Technically, lith ion batteries will last the longest if they are cycled between approx 80% and 40% of their full capacity. Charging them fully to 100% from completely discharged is more damaging than say, charging them twice from 30% to 80%.
That said, I do not know if the phone manufacturers already build some of that overhead into their batteries/charging logic or not. At the company I work for, we do that for our batt powered products, but not completely conservitive to limit the range between 80% and 40%. We'd just be giving up too much usable capacity.
Regardless, plug the thing in when it needs a charge and unplug the next time you get around to it.
-Suntan