After thinking it over, I'm inclined to believe it matters what type of biking you're doing.

You should wear a helmet regardless. It's easy to misjudge. I remember seeing a cyclist at U.C. Berkeley laying on the ground with blood pooling around his head. No helmet. From what I can tell, he was zipping downhill and hit a water fountain. You might think that the middle of a college campus is a fairly safe place to bike given the lack of cars, but in truth, it can get dangerous pretty quickly. Lots of student walking around not paying attention and, in Berkeley's case, steep hills.