I always wear a helmet on my commute or other trip longer than a mile or so away. I always wear a helmet around home, when my kids could see me. And of course on group functions that require it.
I don't always wear one on short daytime trips near my office to run errands or pick up lunch, or ride on a nearby path. Sometimes I just want to rebel against those who consider helmet use the first rule of bike safety. (I think it's maybe second or third, after such things as being visible and predictable and observing right-of-way rules.) And sometimes I just want to feel the wind in my hair.