There's not much volume between the pellet and the breech face. Seating deeper increases that volume a lot. The space behind the pellet is going to affect start pressures and the point your piston recoils on the compressed air in the chamber.
It seems like a sensitive spot to adjust. Gut feeling tells me seating too deep would change the volumes too much and cause problems. As long as a seater got the pellet in there far enough and uniform it would be beneficial. Deeper may not be better.
A stiff pellet seated with a ball would seal up dandy. A skirt expanded too much may cause erratic start pressures. So some pellets may benefit from a seater and some may not. Head diameter, barrel diameter and skirt thickness might change the seating friction and thereby the strategy.
I use my thumb and try to get things uniform as I can. If a guy was serious about it he would test his best pellet with some creatively shaped seating tools that offered a range of depths and skirt expansion. It might make a good pellet better or a cheap pellet best.
...or it might do nothing at all.