Topping off the gas tank can result in your paying for gasoline that is fed back into the station's tanks because your gas tank is full. You need extra room in your gas tank to allow the gasoline to expand. If you top off your tank, the extra gas may evaporate into your vehicle’s vapor collection system. That system may become fouled and will not work properly causing your vehicle to run poorly and have high gas emissions.
http://www.epa.gov/donttopoff/