I always like to check the caps and drains after being fuelled, and it seems sensible to check how much you get fuelled with after a flight, so you can get a rough fuel burn.
Example: the twin I fly burns about 18 gal / hour in general training, and at 65 % power. That means about 2.5 hours are in the mains, and 3 hours in the aux tanks.
Typically the left main gauge indicates 1/4 full when it is really at 3/4s. The right main indicates correctly. I'd be pretty unhappy with this if I was on a long flight and switching back to a low-looking main with a descent and approach to shoot.