Just one suggestion to add that I found useful many years ago, first roll the piston in the cylinder you're working on to TDC, lock the engine in that position by putting it in gear, parking brake on, and then apply compressed air to the cylinder. Then if you accidentally drop a valve in the procedure, it'll only drop a short distance (maybe 1/2" or less, if I recall) before it hits the piston and can easily be pulled back up.
