Some people's systems are virtually flat when they are off, but others' are more concave, depending on their head shape. If you have a concave system and you squash it flat, you may be stretching the outer rim (presumably that is why they transport hair systems on those plastic shapes). Stretching the base is a bad idea, because it will cause wrinkles in your system when you come to apply it.
I agree, I haven't noticed any deterioration on poly rims. I was talking about a thinskin rim on a lace base.