Depends on the bike. For most commuting I would use a mirror. For flat or riser bars the Mirrycle MTB mirror works well also works good on recumbents. For drop bars, the CatEye that fits into the bar end works well. There is also the helmet mount, eyeglass mount and the tiny ones that mount inside your glasses.

MTB: no mirror
Recumbent: mirror, I can't really turn around.
Road Bike: no mirror
Fixie/SS: no mirror

I commute on a recumbent, but will commute on other bikes for variety. I never ride my MTB on the road.