I have never used a screen protector because I know I would screw up applying it.
However not every case is made evenly to protect. There was a case I bought that scratched the back of my 3GS like hell that i noticed after only a couple of days. I searched and found another case that was thinner & padded inside. so buyer beware on any cell phone case make sure it has good protection for the back of your cell phone while using one.