Apologies to the OP for my initial comment. It's just that the helmet question is a bit of an ongoing thing that often becomes a flamewar... (but for those that know the history of this kind of thread, ya gotta admit: That's a funny picture.)
In all seriousness, I wear a helmet on my commute but I'm known to leave it behind when I'm just going across the street to the grocery store. My commute, although mostly MUP, is also mostly in the dark (in the morning) so I'm more comfortable with the brain bucket on my head. My grocery run means cutting through the parking lot of my apartment complex and crossing the street to the grocery parking lot. I'm not too worried about that one because I'm not going far or fast.
If I'm going anywhere else that requires I'm on the street for a while, then I put on the helmet.