really depends on you, how well you take care of stuff, and what sort of environments you're in.
I tend to prefer the peace of mind of a screen protector. On the other hand, I recently used a friend's ipad who doesn't use one and has had it in the Apple stock case since he bought it close to launch last year -- and it looked pristine.
In the end though it's sort of like insurance -- if something happens you'll wish you'd had it.