I'd never force-fill the tank, but aside from that, that's the same way I get my mpg. Some tanks don't need to force-fill. The pressure builds up the same in the tank assuming A) You're on a level ground and B) It's the same temperature outside. But the changes aren't enough to throw the mileage off enough to be seriously noticeable.
The pumps stop when a certain pressure has been reached in the tank. If it's off from pump to pump, it won't be more than 1/10th of a gallon which isn't enough to throw off the calculations that much. It's reasonable to say that your MPG is +/- 0.2 MPG.
Also, you should never top off your tank:
Topping Off