No it shouldn't be too much of a problem. I am guessing you are worried about heat building up inside the case. Evo has enough ventilation to dissipate enough heat for synchronizing or charging. Unless you have your iPod connected to your computer all the time and let the HDD spin to death, I wouldn't worry too much about it. As for other cases, I can't say....Evo has perforations in the back, but I have seen many other cases that has nothing of the sort.
Also, stretching the Evo doesn't really affect it too much. The shape will come back as long as the case isn't heated while it is stretched.