For the time i spent without a screen protector on mine i did not scratch at all. I'd stick it in my jeans with other stuff, and not a scratch. BUT, some how for some reason my brother in law ALWAYS finds a way to scratch his screen, in the first couple of days he had his thunderbolt he got a long 3 1/2" scratch on it. Same thing happend on his rezound, and when he returned the rezound and got a razr he also managed to scratch it within the first week.
If you take care of your devices you should have not problem, I dont think he takes care of his very well because he always seems to scratch his phone, its insane, my 4 year old vouge is still flawless.