Whether or not this should be common sense alot of people do it all the time.
I'm paranoid. I keep the jack under the car, still up and touching the car. Not because it'll keep the car from coming down, but because it'll make lifting the car that much quicker for whoever's helping me at the time. I always put any removed wheels under the closest, solid part of the frame that's not completely in the way. The jack stands go one on each side of my body, but not close enough that if they were to fall they'd land on me. I keep blocks on both sides of any wheels that are still touching the ground.
One more important thing: NEVER rely on the e-brake or transmission being in gear to keep the car from moving, whether the ground is level or not.
I've had a car come down on me before. The one jackstand I was using twisted, the jack wasn't under the car ready to lift. The wheel under the rear subframe saved my ass. I didn't come out of it scotch free, but I came out of it. I had a massive bruise across my ribcage (which stung for months) and a cut across my forearm.
Personally, I think it'd be a good idea to put up a sticky with basic safety measures. You all need to remember that, until owning a Supra, alot of the members here have only ever done the most basic of maintenence. Some haven't even done that. So perhaps before we go telling every nooB that shows up to start doing their own work, we should also make sure they understand what precautions they should take to ensure their own safety.