While I have not attended the vehicle class, I have trained with Steve multiple times. He is not big on fluff or flash either on the square range or inside the shoothouse. The context I have seen a muzzle up or "temple index" being utilized to a great extent is exiting a vehicle without flagging your legs or other people in the area.
We use a muzzle up position in our vehicle classes to provide a safe direction with other "people" in the vehicle with you. Think pushing a passenger out of the way to engage a target out their window, or engaging a target out of the back window without flagging someone sitting beside you.