The phones have a smart charge circuit in them that allow for fast charge, trickle charge once meeting adequate charge levels to maintain charge level and prevent overcharging.
The only thing that I have read and been warned about was using the phone for extended time while on charge. The input/output cycle of the phone attempting to charge the battery at the same time the phone is trying to use energy from the battery during use can, I've been told, cause the battery to swell. That being said, I do make short calls when in my vehicle and I tend to leave it on the charger when driving(using a 5v/2.1amp car charger) and can't really say I've had negative effects to it. I was recently having some strange charging, erratic behavior issues with it, but after taking it apart and finding a ribbon cable not plugged in properly, seems to have cleared that up.