Getting the weight off of the wheels is a good idea. Make sure you support it under the axle and not under the frame of the trailer.
That's the opposite of what I would think- why keep the weight on the axle/springs all winter?? Get the weight off the tires and suspension.
The trailer is designed to be supported by the axle(s) and 10% or less on the tongue. If you think it's important to take the weight off the springs, you need to support the trailer frame at the points where it attaches to the springs.
People who use their trailer boats (or other trailers) year round do not seem to have tire or spring problems. Even if you were to get "flat spots" on the trailer tires (and I doubt you will), they will go away with a few miles of towing.
I wouldn't bother. And I didn't for the years I had trailered boats.