It's tough to get everyone on your team aligned in Stamina, but like mentioned here already, if you have a 7 or 8 ST amongst your starters, it is possible they'll play 90 minutes a week if you play close league games, and that is bad for Game Shape.
I personally like to keep my ST between 3 and 5. I also play a ten man lineup, so they get plenty of rest and my team isn't at too much risk when my bench is in the game. Once Stamina drops to 2 on my lowest starter, I train him back up to 3.