I have the c5 and my son, 2 years old, so far launched the device soooooo many times that I thought I was going to buy a new phone but I was wrong. It's still all one piece and without scratches, I haven't installed any tempered protective glass so yes, it's safe to use without one.
The resons I askes because I am tired buying tempered glass because the the adjective keeps loosing its stickness every 2 months....thanks for the info