To get the most out of your battery in the phone you should charge it and keep the charge between a percentage of 40 to 80 percent.
So before taking your phone off the charger you should make sure it has at least 40 percent to 80 percent of charge to keep the battery charged up enough to use it and to keep it from draining down on you when needing the phone.
I always like to keep my cell phone plugged in when not in use so that it's always charged and ready to go and I haven't had any problem doing so.
Keeping it on the charger when not using it keeps the cell phone at full charge when I need to use it but I try to charge the battery back up on the phone when the battery gets to 40 percent and I never let the phone get to complete empty before charging it as it could make the battery lifespan go down on the battery.