The idea of staying on the vertical axis is fine for beginners and recreational players. I know of no accomplished player that tries to stay on the vertical axis. I know of several who say they stay on the vertical axis, but watching their game it's obvious they're not. I have done hundreds of hours of commentary for professional events. There isn't a professional player that doesn't hit most shots with some kind of left/right spin, and the players that people on these board will stay "closer to center" are the ones that really go to extremes like Buddy Hall and Chang Jung-Lin.
I was sitting next to an internet poster who had never seen the elite professionals live, and he literally gasped when Chang stroked a shot with extreme miscue-approach English. And the cueball wasn't even going anywhere special. . And we had this discussion. He always thought Chang stayed close vertical center, but watching up and close and personal, observation trumped all. I also mention Buddy Hall because it amazes me how many people say he stayed closer to center, yet Buddy not only taught the Clock System, but he discusses the importance of being able to use all of the ball in one of his post match interviews after maybe a US Open event on YouTube.
Freddie <~~~ a little off center