I only use those film type screen protectors, not the tempered glass types. Tempered glass are sometimes as hard as the screen itself, but as with the screen then they can also crack.
Film types, just as with tints, helps to prevent cracking, just like when you drops the phone in one of its corners, or face down. That's why most times you'll see naked screens with cracks running their length (or diagonally) or cracked in one of their corners. It can be quite ugly after a while though as it's not as scratchproof as the screen or a tempered glass. That's why I prefer those flip covers.