I use a mirrors. On my tamdem I use a Helmet type and on my commuter I use my bar end. I can't ride without one.
On the Tamdem I like to keep to the right of the path when faster cyclist come up from behind or when I pass other cyclist. On the commuter I use it to work my around slow cars and to keep right for faster cyclist as well. Some of the cyclist I come up on that do not have a mirror must have their reason, but most do not move at first when I call out or ring my bike bell. Mybe I just like to be courteous.