From my own experience and that of just about everyone else on here that doesn't use a screen protector...you don't need one.
I used to be very careful when it came to my Storm's screen w/o a protector on it, but after months of use and not even the tiniest little scratch, I just stopped caring and it's still in perfect condition. I also have both my traguses pierced (
visual reference), and with every other phone I have owned, a dull scratched up spot has appeared over time on the screen from the jewelry rubbing on the screen while talking. My Storm doesn't even the slightest hint of one.