In my experience, the only thing that "Let the coach decide" does is mess up training minutes. I don't like it at all. If you do follow the depth chart you don't really have to worry about reserves not getting enough minutes because they can get most of the minutes in the scrimmages. Generally your starters will get 36-42 minutes and your reserves will get the rest. There are other factors like stamina and whether or not the game is a blow out.