The laptop knows how to take care of the battery. It will engage actual charging only when needed. Nothing bad will happen if you keep the adapter plugged in.
DragonLord's advice is spot on if you want to preserve the battery as much as possible. Technically the best place for a Li-Ion battery to sit at is around 50% charge. But then what's the point. That's getting too OCD for me. The purpose of a battery is to make a laptop portable. It also serves as a kind of UPS, so that if there's a power outage, your laptop doesn't just shut off potentially corrupting data.
So go nuts, leave it plugged it to the wall, it's all good :-)