There are two schools of thought on 'protective' filters. One is that they should be fitted to all lenses at all times because you never know what might hit the front of the lens, the other that the main thing they protect against is image quality.
Guess which camp I fall into
The only time I use a protective filter is if there's a real chance of something nasty (mud, sand, salt water) coming into contact with the front element. And then, only if I can find one. I feel a lens hood protects equally well against accidental touching and minor knocks and even falls. My D80 with 18-135 and lens hood fitted felll off a chair. The hood broke, but everything else was fine. My D300s and 18-105 went on an unplanned trip over a concrete car park, with a couple of scratches to the lens hood.
edit: Use a lens hood, be reasonably careful, and fit the lens cap when you're not using the camera.
Chris