My wife and I both have Thermarests which we've used in the most god forsaken territory you can imagine on mountaineering trips. They'll turn a scree slope into a feather bed! If they start to deflate, you have to persist with the bathtub until you find the leak, then fix it with the gunge that comes in the repair kit. This has only happened once to each of us after years of abuse. As long as you leave it to self inflate when you store it it'll be fine.
Cheers
Ray