It dosen't harm your device, it CAN cause glitches in the iOS based on device, generation of device, and just what firmware it is but Hardware is safe so if something goes wrong you can ALWAYS restore. Also let me be clear, IT DOES NOT VOID WARRANTY* Apple wants its customers to stop jailbreaking so they freaked the masses out by saying it does. Reality is unless you mess with the baseband to unlock an iPhone a clean restore clears everything and they have no way to know you did anything to it, and a baseband change is really the only grounds for voiding the warranty in this case. Besides, a clean restore almost ALWAYS fixes the issue, as in 99.99999999999999999999% of the time.