It depends what you are working on! If I were working on the axle, I'd put them under the chassis frame (or jacking points on a monocoque vehicle), but if I was working on the shell, brakes, diff etc I usually put them under the axle tube.
I generally either use specific jacking points, or try to position the stands as close as possible to the suspension mounting points - these, by definition, must be string enough to support the weight of the vehicle. The most important thing is to make sure the stands are secure and level, and that the weight is evenly spread over the pair, and over each leg of each stand, so there is no chance of the car shifting or moving on them - make sure to securely chock the front wheels as well!