I fill up until it goes *click* and then I give it one more *click* to make sure it was actually full.
My best friend's family all drive Japanese vehicles. They purchased a new Honda van a while back and, like he always did on their previous vehicles, my friend's father topped it off/put as much as he could in. Then started down the road.
After a few miles, he soon realized that the fuel gauge was dropping like a rock and, as he happened to be near a Honda dealer, pulled in.
The mechanic came up to him a few minutes after bringing it in, "Sir, do you fill up the gas tank all the way to the top?"
"Yeah, how did you know that?"
"Because your gas cap was completely blown off and you lost half a tank of gas."
Because of how vehicles pressurize the tank these days, you really should stop at *click* and not try to put as much in as possible.
I wouldn't be surprised if your overall MPG dropped.
http://action.publicbroadcasting.net/cartalk/posts/list/1575409.page
Also check out
http://www.epa.gov/donttopoff/
http://www.sbcapcd.org/edu/dont-top-off.htm