Be really careful when installing and calibrating them when there is little to no fuel in the tanks. Some less than professional installations misleadingly show some fuel remaining when there isn't any. You must ensure there is no residual fuel in the bottom of the sight gauge at any attitude after any climb, descent, slip, bank, well before the tank is empty.
In my experience it is better if the gauge shows the tank is dry 20 minutes or so before it's empty.