All excellent responses. There is no good reason to overfill a fuel tank. The most I will round up is to the next nearest 5 cents, never to the nearest gallon. If you want to track your fuel economy, the best way to do it is to record every fillup in a small notebook kept in the car. I label columns as so: date, odom, trip odom, gals, $cost and mpg. At each fillup, I enter the data for the first 5 columns and leave the last one blank. When its convenient, I divide the gallons into the trip odometer and record that mpg calculation in the last column. I have done this for many cars starting with my first 42 years ago - and have gotten my wife and kids to do it. It quickly becomes an automatic part of the re-fueling process. Since I usually fill my car every other day, there are plenty of entries. Keeping a notebook is also a good way to record oil changes, tire rotations, wiper blade replacements, etc. When I sell one of my cars, the notebook goes with it. I can't think of any negative doing this.
On our fleet vehicles at work, we have large printed warnings "DO NOT TOP OFF" to prevent damage to the emissions components.