I would say boo on forgetting the helmet, but it has happened to me. As for making a conscient choice, it really depends on the kind of riding that you are going to do, and the risks that you accept and assume. I wouln't go to work with out a helmet, but I cruise close to motor vehicle speeds. However, on a FTWS (Faster Than Walking Speed) bike to go to the corner market, I would agree that a helmet isn't needed all the time.
So for me the rule is generally: Helmet and clipless for "playing in traffic" and platforms and whatever I am wearing for FTWS.
Hah, you Canadiens, 18˚C, and you call that a heat wave?