If you unplug the charger the moment the phone hits 100%, you won't have as much charge in the battery as if you leave it plugged in for an additional 30 minutes. (But it's a bad idea to charge much over 90% - the battery gets warm and can "outgass" some of the electrolyte, killing it just a little on every charge [in addition to all the other things that kill it a little every charge].)