"The folds will create creases in the bladders and over time can cause them to wear prematurely."

The above advise is very good and important! If you fold the bladders they will go bad at the creases just like an inner tube that has been left folded or like the rubber hippers did at the creases where they were folded. Your best bet would be to just deflate the pontoons some and find a place to store them partially inflated so that no creases will be made.

You have received excellant advise from DarrinG.