Place I hire from I check the paperwork and the gauges and dip the tanks. If all three don't agree that I've got an hour reserve I fill up. To full tanks. In East Anglia, where you're never more than ten minutes from a runway and can land on pretty well every field.
All I've ever heard from an instructor is "good decision", even when there was almost certainly more than enough fuel in the first place.
If I ever found "disagreements" with the CFI over insisting that I fill right up I'd fly somewhere else instead, simple as that.