I've used TPU cases on my 3GS, 4, 4S, and now 5, and none of them have damaged a thing, and I've used a wide range of them, from the Incipio NGP to cheapie 99-cent ones from eBay. I guess I could see how if something were to get between the case and your phone when putting the case on, like an abrasive piece of dust or sand or something, then sure, that could lead to damage, but I'm careful and have never suffered any ill effects.