Mine is also a short (15-minute) commute, so there aren't too many variables that would make me late if I hit a snag on the way. Regardless, I always leave about an hour early so that, if some whim hits me to take a detour or stop for breakfast, I can still do that with no stress at all.

I think whatever attitude about time prevails in the rest of your life probably just carries over to your bicycle commuting habits. For me, the risk of being late for anything, whether it's work, a lunch date, a movie, anything, produces much more stress than I want to deal with at this stage of my life. Leave early, pedal comfortably, love every minute of the ride.