I use my display.

First off its not thrown off any more or less than the odometer is by tires.... and you're using the odometer to determine miles right?
Second, the engines computer is MUCH more accurate at determining the relationship between fuel consumed and miles driven. The resolution for fuel consumption in the engine is very refined..... however the "trigger" for gas stations are not. Just because you fill up and the gas pump turns off at 16.56 gallons does not mean that you have used 16.56 gallons. So dividing "miles driven" by amount to fill up is actually much more inaccurate. Some gas pumps are more sensitive than others so they'll stop sooner and therefore your "calculated" MPG will be higher than actual.

The computer however is very consistent and there gives the best tank-to-tank comparison of gas mileage changes.

That's how I look at it anyways.