It's a good idea to use the pull starters when up by at least.3 scores in the 4th quarter.
Why?
Injuries in MFN can be set via a slider by the league administration and to make that work, it means there are a set number of injuries which must occur in a given week across all 32 teams.
So the rub is that when the "random" dice rolls during all 16 games don't reach that magic number, the sim forces the minimum number to occur at the end of games.
I admim 2 leagues that are well past season 30 and it's the most common complaint among owners. "I was winning 50-0 and my QB/RB/WR got hurt in the final few plays!
The final 2 minutes of the game are deadly! Protect yourself if you can!