When your phone is completely charged, it switches to a trickle effect. This means that it'll draw only as much power from its power adapter as it needs to keep its full charge. Your phone still expends energy as long as it's on, even if it's plugged in. Whenever the battery is fully charged, however, it simply stops charging and will suffer no ill effects. Your battery will probably degrade from the effects of time before it experiences any issues from charging overnight.
So in turn, you are fine to keep it plugged in, even after it's charged, I always unplug mine personally just to be safe, but that's just me.