How it is decided to add the fifth man to a game?

The schools (at our urging) ask for the 5th and in some cases a timer. All 5 on the field get paid a full fee. There is a separate timer fee. We started this about 5 years ago and the numbers increase each year.

I don't have any experience with 6-man or know of any using this mechanic in our area.
In Oklahoma we use 5-man mechanics on every varsity class level except 8-man games, which is 4-man mechanics. With today's passing games this is sometimes difficult to get all the eligibiles accounted for. I would think that a 6-man crew would have a little easier time of it. Currently there is a push to cover the semi-finals and championship games with a 7-man crew. After working some college, 7 is the way to go. Man, that is sweet!
BTW, we use 4 on sub-varsity games.