Originally Posted by KiLLeR2001

Because you still do not know if its a sensor issue or you not filling up to the appropriate amount. If you start to overflow, stop pumping fuel... It's also important to be sure you are on a level surface.

edit: Look, if you want to be on the safe side. Wait till you are 1/2 tank exactly, fill up 4 gallons and the needle should be pointing directly at the 3/4th mark. There will be a little variation but this is the best way to test its accuracy.