ugh, I almost fell for that preshot routine stuff lol...Poolnut, I respectfully disagree. I think anyone who actually counts practice strokes has a form of insanity.
I immediately googled efren to see if he really does always pull the trigger on the 2nd swing.
http://www.youtube.com/watch?v=LeJbyrUC0zI
You can see that on the 2nd shot, which is a harder cut, he takes more swings. Efren's magic is NOT in his preshot routine.
Preshot routine is overhyped. The more focus you put into unimportant things that ought to be natural and subconscious (breathing, foot position, practice stroke count, where your eyes want to focus) the less focus you can put on lining up and stroking the shot.
http://forums.azbilliards.com/showthread.php?t=140076
Check out posts 5, 8, 22, 41, etc. See a pattern? The point is, excessive thinking about this shít will cause you to utterly shark yourself. It should be as natural as walking, not carefully counted and calculated.
To answer the original poster's question:
Shoot when nothing else is bothering you about the shot. You aren't worried you may be lined up to overcut. You aren't worried about undercutting it. There's no question about whether you're going to hit the part of the cue ball you want. You've compensated as best as you can for throw, squirt, etc. You like your general gameplan. You have a few warmup strokes under your belt to judge the speed.
As long as something is nagging at you and you're worried the shot might not work... even if you're not sure why... you don't pull the trigger. You honestly ask yourself what's bothering you, see if you can correct, and if necessary stand up and change your whole shooting plan.
Once nothing at all is preventing you from pulling the trigger, why wouldn't you? Are you trying to bore your opponent? ;D