That's what i've always kinda been confused with; 'overcharging'....
Whether or not it's ok to leave a phone charging well past the "100% charged" time it normally takes...such as leaving it charging all night, etc.
And over the years i think i've read a bazillion DIFFERENT answers to that...so i really dunno exactly what to believe.
My older company phones, over the years, were Nexus, Motorola, and Blackberrys, and all of them saw heavily reduced batteries by continually charging them...'topping them off'....to the point that they'd be totally drained with just 2-3 hours of usage....so with my personal phones (Razr Maxx, Iphone, Incredible 2, Eris, etc) i've always done the 'charge all the way up, then use all the way back down to about 1%, then recharge all the way back up' thing, and i've had great luck with that. and since my Moto X is only 2 weeks old, i'm still trying to determine what route to take.