Not really in Australia, our winters are some of the best times to go sailing, unless you are in Hobart that is!
Very different in the Northern hemisphere though where the water freezes and you have to take your boat out of the water, remove the rig and put anti freeze in anything that uses water. 
That is where the Vacuwash process developed due to the sheer volume of sails that needed cleaning prior to winterising. Boat owners there know the benefits of a yearly sail clean and it is in their yearly boat maintenance schedule. 
Over half of our business is from returning customers who also know the benefits of a yearly sail clean.
If you aren't going to use your boat over winter, then at a minimum get the sails off the rig and put them below, or better still send them to us for a wash. The same applies for your canvas.

This poor owner didn't get all his winterising finished off before disaster struck.

Vacuwash exhibits at many boat shows around Australia; here is a cheesy interview from Sail Expo at RPAYC

Hot in January, hot and wet in February, wet and humid in March. As much as you probably have had enough boating over the summer months, the recent weather we are having on the East coast of Australia will give you the excuse to get back to your boat and dry her out. For many it is already too late, I heard a conversation on the club tender the other day, 'the engine was half underwater as the bilge pump stopped working' there is always something on a boat!
A sail came in last week to be cleaned, and the owner said he had only left it for a month, i'm afraid that that is all it takes in this climate.
If you are not going to be using your sails for any more that a month, then take them off the rig and store them below, and off the floor to just in case the bilge pump stops working.

​Sail below, clean to crappy in just one month, and yes the sail is spotless again after Vacuwashing.

Last weekend Australia's east coast was smashed by an what is termed an 'East Coast Low' other parts of the world they are Cyclones or Hurricanes, whatever they are called the results are usually similar. 
We haven't had that much rain since the last time we had that much rain, and records were broken for the number of stories/news programs talking about records being broken.
So we have all dried out our garages/wet carpet, cleaned out the green swimming pools, grated the fallen trees, most households are back to normal, all except for boat owners!
These guys will be down on their boats this weekend, taking advantage of the 21C winter weather to dryout their boats. Boats leak, even new ones, there are no open windows, so that damp interior will be going moldy as I type. 
Any sails stored below will need to be aired, as will bunk cushions and anything/everything else. 
Then there are the sails furled around the forestay, or stacked on the boom. These will still be soaking wet even a week after the rain.
Only one thing to do, Go Sailing.
Any boat that doesn't go sailing this weekend will be a Vacuwash customer within the next couple of months, that's all it takes for those green stripes of mold to form on your nice white sails.
If you don't want you sail to look like the one in the picture, get down to your boat and go for a sail.
See you on the water, I'll be out there.
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Today I just cleaned what should be a perfectly good jib. When I say 'should' I mean that if the sail is not that old, its cut is an expensive tri radial, the sail material is a top of the range laminate. The sail though has been left on a furler, in the weather and the sail has now gone to crap. I can make the sail look better, yet once the UV strip starts to break down, then the sail it is there to protect also breaks down. All this happens while your unloved boat swings at its mooring as you attend to other pastimes, it also happens a lot quicker than you think. 
I'm not going to get into the furler vs put away headsails here, but here are the pointers if you are going to furl your headsail.
Air and dry the sail weekly even if just at the dock in calm conditions.
Dacron is a much better cloth to clean, so furl that please. Exotic laminates are great sails, yet I dont think they should be furled.
Make the UV strip out of 100% acrylic canvas, it lasts better than sticky back and looks better with age, and also furl it the right way so the UV is on the outside!
Make the cut a simple cross panel, tri radial sails cant be cut down to remove any leach that has UV damage.
Lastly, if your sail is small enough and you can handle it by yourself, then take it off the rig and put it below, it will last forever.
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This is what 18 months on a furler will do to your sail. At a rough guess this sail is worth $5000, yet when it came to me it was as you see below. What you can't tell though is the stink, the amount of water in the sail, or see the little critters that ran out as we unrolled it!
A couple of points come to mind here, firstly if you are not going to use your sails for a length of time, then get them off the rig and put them below out of the weather; a length of time would be in my opinion (and remember we all have one) would be a month. Secondly, this sail is a laminated one, the fabric is fantastic in strength and shape, yet sadly sailmakers do not recommend getting them wet, go figure that!
As soon as the laminate is sewn, the needle leaves tiny holes for the water to wick through, thus you see mold appear in the laminate. 
So what to do; laminate sails should be sprayed in SailkotePlus when new to help prevent the mold appearing in the first place, and you need to go sailing a lot more often to keep the sails dry.
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​I'm guessing that boom bags have been around for 30 years or so, prior to that we would remove the sail off the boom, take out all the batterns, flake, bag it and put it in the cabin. Then something happened, well numerous events even that led to the boom bag as we know it today. These include larger boats (larger sails) less crew to do the work (post RBT weekend sailing has never been the same), full length batterns, flashy bat cars that attach to the mast which need special tools to remove, lazy jacks, and plenty more I'm sure. The result is a main that now lives on the boom and hopefully under a canvas cover of some sort.

There are two main types, one with lazy jacks attached as per the main picture above, which the sail falls into and a zip on the top to enclose the sail when back at the dock.  The other is a cover with the zip underneath that you put on the boom once the sail has been nicely flaked on the boom. I'm ignoring in boom/mast furlers this time, that is a full comment in itself.

As a sail cleaner, I can tell what type of bag you have and what condition it is in, and wait for it, an opinion on which bag/cover is best, I think you already know what I'm going to say.
Zips fail faster than you can get them repaired, zips are also not waterproof, so if your zip is on top of the boom, your main will be wet everytime it rains, it will go mouldy and you will have to call Vacuwash to clean it, remember it is also too big, which is why you left it on the boom in the first place. Also at the clew end there is the perfect hole for 'birdy numb numb' to squeeze in and produce more birdy numb numb's, one week is all it takes! A sail can be brand new, and in a week can look like, well s*it.
My suggestion it to attach the lazy jacks to the bottom of the boom, sailtie the main to the boom itself, move lazy jacks forward, then cover the sail with a one piece cover, zip or clips under, making sure there are no holes for BNN.
Another huge advantage it that you dont have that huge bit of canvas flapping while you are sailing.
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​The sails below were home to BNN for a short time, think what fell on the owners heads as the sails were raised. (both sails cleaned up beautifully)
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That headline is usually the first comment I hear when a boat owner calls me.
Invariably the sail has suffered from lack of use, left furled on the forestay or sitting under a leaky boom bag. 
It doesn't take long, under a month you will start to see the mildew in one of its various forms.
To prevent the mould from starting, you need to air your sails ie go sailing, at least weekly, even a half hour run will do your sails the world of good, if not yourself.