Never, *ever* just turn off the switch. Somebody will, without fail, walk in, hit the switch, and shock the living crap out of you.
Or, you will leave the room for something, and hit the switch yourself, on the way back in.
The wall switch either interrupts the black wire (hot) or the white wire (neutral). If it interrupts the white wire, you've still got 110 volts coming straight toward you, from your friendly breaker panel, just waiting for you to kindly complete its circuit between it, your happy little fingers (if not your entire torso) and the ground wire, assuming there is one. Don't ask me how I know about this.
