Woah, settle down...

The "Proper" way to fill up the tank is to wait until the gas pump clicks off by itself, then just put the gas cap back on ;-) You might hear people around here refer to that as filling up to "the first click".

Sometimes when filling the tank up to the first click a small amount of liquid gas will get into the vent for the evaporative emission canister, a small amount isn't something to be worried about. This will artificially inflate the FE for a short period of time after filling up, I usually see it for 10 miles or so. The thing that could be damaging is to keep putting in more and more gas after the first click (or 3rd, 5th, 20th, etc), some people actually put gas in until it is about to spill out of the filler neck... that can damage the car as the gas system was not designed to operate that way...

If I were you I wouldn't worry to much if I were you unless you see the FE pegged at 120 mpg at times you wouldn’t expect and for extended periods of time (like over multiple days), or if the check engine light turns on as that could indicate if there is a problem with your car's emission control system.

Hope that helps,
~Droid