A Li-ion battery needs to be regularly cycle through charges to hold their capacity for as long as possible. If you always keep the battery at 100%, then you?ll slowly begin to notice that the battery won?t last as long away from the outlet. Leaving it charged overnight and then using it during the day seems to be the most practical and safe way to go about it.
Heat may be a concern. The single worst thing you can do to a Li-ion battery is keep it at 100% charge and exposed to high temperatures. It will compromise both the lifespan and capacity. Eg, if you play a graphics intensive game while keeping the device plugged in at 100% charge.
But don?t panic. Your battery is not going to suddenly die overnight on you. It takes prolonged use under these stressful conditions for your battery to have any noticeable effect. Nowadays, batteries seem to be able to take quite a bit of punishment. And if you?re going to replace your phone in a year or so anyway, this may not even be a concern.