It should discharge 3% then back to full then back down. It's perfectly safe to leave it plugged in overnight. In fact it is better for the battery than letting it run down through the night.
I always leave my phone and tablet plugged in until I need them. This is something I've done always with no issues. Since batteries are measured in charge cycles and with heat being a batteries biggest nemesis, it only makes sense to me that the less you charge it the longer it will last. IMO