I've used wood, but I have a lot more comfort knowing that the car is being held up with something that was specifically designed to hold the car up.
General car maintenance safety:
Always engage the hand brake before jacking up the car (unless you're doing the brakes (and then be very careful)), work on a flat level surface, put the wheels under the car as a backup, just behind the wheel well. I usually also leave the floor jack jacking up the car as well, as a tertiary safety measure. The last thing I want is for that car to come screaming down at my face!
I've had jack stands slip on a non-level surfaces when I forgot to set the hand brake... scary on so many levels. When the wheels disengage the ground the car can move +/- 1" or so in either direction because of the way transmission is geared (I believe).