Press the power switch once and it will go into standby. Press it again and it will power off. The only difference I have noticed in using the power switch versus closing the lid is that using the power switch will require you to enter your password to regain access whereas closing the lid will not (assuming you have set up security to require a password).
At work, I use the power switch, at home I either close the lid, or accidentally let it time out. I only close the lid at home because I don't bother taking the charger home with me. The battery will last through a long weekend for me.