I put them under the rear axle and under the front lower control arms. I use very heavy duty stands. Life depends on them. If you need the suspension to "hang", then place them under the frame just behind the front wheels and ahead of the rears. Make sure the car is setting firmly on the stands before going under it. Lost a friend because his car slid off some cheapo stands.
Use the frame. Don't want the car bouncing around and slipping off the stands. Might be OK for rotating tires or something but I'd never do anything extensive. Besides, how the hell you supposed to do suspension work if the car is still sitting on it?
