Nothing bad will happen to the cell phone if left plugged into the charger all the time even when the battery is 100% fully charged.
The only reason the phone recommends you unplug the cell phone charger once it reaches 100% is to save electricity but the amount of electricity your cell phone will use while charging all the time is very little and you shouldn't notice any difference on your electric bill whether you leave it plugged in or unplug it.
I personally just leave my cell phone plugged in all the time when at home or when I'm not using it and am near an electrical outlet.