If you are running one of the old classic valves with the X on-off top, you may also have one of the backs that has a hole that does not line up with the hole in the front half. Its easy to check by screwing the valve apart and marking the position of the holes on the outside of the valve and then screwing it back together. If the holes don't line up, it is easy to drill a new hole in the back half that lines up with the one in the front half. You just have to make sure you do it on an angle so that the hole goes into the front of the regulator piston area and not beside it. By lining up the holes, you will reduce any potential valve starvation. If you have one of the newer classic valves, the indexing they did when cutting the threads allowed them to all line up properly anyway.
Except for the Automag in front, its usually the man behind the equipment that counts.