When I get under a car, I always obsessively contemplate what it would be like if the thing were to fall...and then ask myself why I thought that.. SCARY! If you need convincing to use jack stands, just jack up the car, get under there a good ways, and imagine it failing
Little tip about chocking wheels... I never had much faith in little wooden wedges. I use a simple pair of cinder blocks. They have weight to them, friction, and the angular bit against the tire does not encourage any chance of the tire "rolling" up it.