I wouldn't worry too much about it. After all, the thermarest come tightly rolled up from the factory, and who knows how long it's been sitting around like that. However, if you haven't noticed, it takes forever to self inflate after keeping it compresesed for a while, if it ever self inflates. I notice it takes 10X longer to inflate if i kept it rolled up for a long period, as opposed to a overnight. I don't think it causes any permanent damage though. I follow the manufactureres recommendations and keep it inflated with the valve open, that allows any moisture from my breath to dry out inside too.

Also, the backpacker models with the die cut foam are even more suceptible to this. The one I have is halfway between an air matress and thremarest, because it will never fully inflate on it's own.