I meant to answer that question but forgot. In all cars except the Prius, the tank is topped off at basically the same point, but the gas pump automatic shut-off does not happen at the exact same point every time. This can result in some tanks' calculated mileage being a bit too high or low, but it will be balanced after the next tank.

So you might drive 500 miles over 15 gallons but the pump shuts off after pumping 14.9, thus making your mileage calculation a bit higher than it really was (33.6 instead of 33.3). In other words you used 15 gallons but you pump 14.9 so you mistakenly think you only used 14.9. But this will be balanced on the next tank since there's less gas in the tank than usual, so you might drive 497 miles over those 14.9 gallons, then pump 15 gallons before shut-off, making it look like you did 497/15 (33.1) when you really did 497/14.9 (33.3) so it balances out.

To sum up, your lifetime mileage will be perfectly accurate but mileage for individual tanks might be off a bit -- slightly high on one tank, then slightly low on the next, balancing out.

But remember: topping off is bad for the environment. It causes unburnt gas to vaporize or even spill (which is REALLY bad). There is a reason both the pump and the car's gas cap contain warnings not to top off.