This article points out why it is not a good idea.
I suppose it makes a difference whether you are talking about a modern car with a filler neck well above the tank or a Model T. Stick the gas pump nozzle into a T tank and fill till it clicks off, you'll be lucky to have a half a tank.
Not all pumps are the same. CA has the recovery fuel pumps. That is why there are two hoses connected to the nozzle. If I know I am about to jump on the highway and drive for the next hour or more, I fill up the neck a little. The truck doesn't have a coal can and it will just spill out. I stop before it spills out. The gas stain on the side of the truck just drives me nuts.
The owner's manual that came with my 2007 Honda Element said not to top off the tank because it might foul the vapor canister and cause the "check engine" light to come on.
While I'm on the subject of the "check engine" light, if you get a light, but the engine seems to be running fine, check to make sure your gas cap is tight, some cars will light the light if the cap is loose.
I don't believe anything the EPA says without independent verification they have been caught in too many LIES! That being said if I am filling up and then going home and parking the car in the summer 1st click is enough.... if I am on a road trip what is in the neck is burned off in 15 0r 20 miles MAX so no harm no foul to put a little extra in. JMHO
One more caution for those who heat their garages in the wintertime - When I still lived in Connecticut, I used to have a 1969 Plymouth Valiant two door sedan. One day in the dead of winter I stopped to get gas (did not even top off), and by the next morning, the cold gas in the tank had expanded in the heated garage and some flowed up the filler pipe, out the filler cap (I guess the gasket was bad), down the side of the car, and onto the floor. Lucky thing a spark from the electric heater fan motor didn't light things off!
Here is California in most counties they have a retractable seal on the nozzles. You must hold it back manually, or you will only get your tank 3/4 full.
Since gas has went down, it was $1.97 in my town today, I fill up to the top every day before I go to work. When it goes up I'll have a full tank.
The only reason to not top off a Model T is to leave room for a little Marvel Mystery Oil. Whoops, did I say that?