All of the gas stations that I have used seem to be flat and level in the pumping area. With the gas nozzle fully inserted, the slant of the vehicle will change the effective volume (the point where the pump shuts off). I think the dash FE will be the more precise number. However, the calculated number (over many tanks) will be the more accurate number. The calculated FE will let you calculate a %error for the dash FE. So far, my dash reads 1.9% low.

It could be worse. The Prius has a bladder style gas tank that can lead to large tank to tank variations in calculated FE.