Are new valve locks necessary

If you bring the piston to TDC , once you put the air to it wouldnt it push it down to BDC ? You can do it without using air so the valve wont drop in case of air failure. Take cylinder your working on to BDC and insert a small piece of rope about 4or5 feet long into the spark plug hole, leaving some sticking out of the hole. Slowly bring piston up to TDC until it stops, the rope will hold the valves up, change the springs, move piston back to BDC so you can get the rope out and then on to the next one.