There is nothing wrong with pulling the springs without pulling the heads. Pulling the heads to do it is not necessarily "doing it right".
Air pressure can be as low as about 75psi or so, but just leave it at whatever pressure you run your shop at (125 or so). Also be careful not to tap the valves with the springs removed. If it breaks the seal on the seat, the valve may drop, and you will be pulling the head then.