Can't really provide any insight into the whole "touch of" topic. I don't adhere to the method. I was simply playing the devil's advocate against playing center ball.
I personally don't fret over tip placement deviations on the CB as it relates to stroke mechanics. It's not that I believe I have flawless mechanics. I just know that it's the one aspect of my game I can count on to be reliable. I also tend to play with more english than you're average joe, so following the 'touch of' logic for sake of compensating center ball misfires is moot in nearly all cases. If and when I play center ball is generally to maintain the initial carom angle to the left or right, so they're struck with a level of force to provide friction induce CB rotation. Any horizontal deviation of a center ball CB strike doesn't generally effect these shots negatively.
All that said, I think the 'touch of' argument is a sound one.
Thanks for a nice in-depth answer. I guess I still have to find a practitioner of "touch of" to see what they say. While the "touch of" argument intuitively seems sound, it won't hold up to scrutiny. The shape of cue ball and tip, the mechanical properties of both, all indicate that error is less punishing the closer to center the hit. Aside from that, we have something to align us, we judge symetrical on each side well.
Had to go on a little road trip and with nothing better to do I noodled this around a little bit. The way we apply side matters, I use a parallel shift although it is set down in place to begin with. Error is less costly with parallel shift than the same error is with back hand english. Even front hand english has less effect.
Something else, Unless several of us are trying to make a shot work I can't remember the last time I noticed tips of english. I want the cue ball to stop over yonder, I want to pocket the object ball, I'll hit the cue ball here to get things started. Ask me how much english I applied and I will probably say something like a lot of low and a little left. Kinda like my cooking. No matter what the recipe starts with there is usually a little windage involved along the line.
Hu