The short answer is you want players to be with the better coaches, regardless of where the coaches rank at that level. In other words, if your AAA pitching coach has 75 pitching IQ rating, your AA pitching coach 67 and your HIgh-A 60, you would want AA pitchers to be with the AA coach rather than the High-A coach because of the rating.
When hiring coaches, take into account the patience and discipline ratings. Patience helps with coaching younger players, while discipline is for the older players. So you would want coaches with good patience in the minors and good discipline in the majors.