Is it okay to fold my winter suit and put it way like that for the summer or would it be better to hang it?
it's definitely better to hang it up in the corner of your closet or whatever. but it can't really hurt to fold it up... it will just probably have a undesirable crease for quite a while once you start using it again
If you're going to fold it, try and put something cylindrical in the fold to prevent creasing (i.e. fold it over a piece of pipe insulation or something). If you're going to hang it, hang it on a wide hanger across the middle (like you would hang dress pants).
I would imagine a crease would weaken the rubber over time, especially if it's being stored like that for long periods.