All devices made within the last few years have circuitry built in to prevent over charging. Most of them will charge at full power to about 90%~95% and then trickle charge to 100% at which point it shuts off the charge. If the phone sits long enough to drop below 95% it will trickle charge back up to 100% and so on. Actually LiIon batteries perform better with small incremental charges rather than letting them discharge completely. (The same circuitry will prevent the battery from a complete discharge, since that can render a battery inert,) It should be perfectly safe to leave it plugged in.