When should you drop foul balls?

Is there a time to drop foul balls? Is that time so predictable that you can teach an exact moment or is it something that is instinctive to an outfielder?

I believe it is a combination of both. But, it is certainly something that can be expressed to outfielders as to when they should consider doing so.

I belive that when the infield comes to an "in position" the outfielders need to be aware of possibly dropping foul balls. If the coach feels that the infield in is a necessary option than the coach believes that cutting that run down is a legitimate strategy. Therefore, on a foul ball that might score the runner at 3B, the outfielder must give some thought to dropping the ball.

If the infield is in, the coach is playing to prevent that run from scoring.

If the ball is caught in foul territory and the runner from 3B can tag and score, then the objective is not met. A run has scored despite an out being recorded.

My premise is that when the infield comes in an outfielder should then have this thought cross his mind. If the ball is shallow enough where he believes he can catch it and still throw the runner out he should go for it. If the ball is too deep where he can't keep him at 3B he should drop the ball intentionally.