it depends on the design of the notebook. some notebook will charge the battery until it is full and STOP charging (idle) in that case, leave the battery in the notebook is not a problem... ( for example, Dell)

however, there are some notebooks don't have the over-charge protection, so when the battery is in the machine, it will charge all the time regardless it's full of empty.. in that case, better take out the battery... if you leave the battery in the machine, i think the battery will die in a year or so... even you don't use it. ( most notebook are this type, including Sony Vaio)

you can check which kinda charging system you have simply go to power function... if the battery is 100% and still says charging.. then you have the second one.. (the first one will apear "idle" when the battery reach 100%)