Aren't we all running on the battery, and plugging it in is merely keeping the battery topped up?
That said, I plug it in when it's dead or nearly dead, and unplug it when the charge light goes from orange to green. I may be wrong, but I subscribe to the belief that it's best to keep the electrons moving, either in a charging or discharging state, to prolong the battery life, as well as the charger's life.
My friend believes that it's best to try to keep it plugged in as much as possible, and she's had numerous failures of both the battery and her charger (2006 MacBook Pro). I don't suppose it matters, though, since the Apple Store just gives her new batteries and chargers every time they stop working. Not sure how that works now that AppleCare is over, though. I seem to recall they still gave her a new battery last time, even though it was out of warranty.