If you restore, the phone goes back to the way it was, as if you had never done anything.
There are apps that crash, but they don't "crash the iPhone." They just exit.
Whether it voids the warranty or not is a legal gray area, but apple seems to be taking a neutral position on hacks, except for carrier unlocking. Clearly, if a hack causes the problem, apple won't fix it, but so far there doesn't seem to be any indication of hacks causing malfunctions. Still, it would be a good idea to restore the phone before taking it into service, otherwise apple may say that your malfunction is caused by the hack.