The downside of overfill...
From what I understand, just let it be, don't over fill. Otherwise, it might cause the gas to fill up the space around the bladder and ruin the vapor canister.
When I pick up Woody last month, the sales rep warned us not to over fill, the service tech warned the same thing, also at other message board such as http://www.greenhybrid.com, also has posts relate to this topic. I believe it because replace the tank is very expensive.
Getting More Volume at the Pump
Unfortunately it seems that every pump's shut-off is different. Even at the same gas station. Trial and error is the best means to find the right pump. Driving a Prius has made me loyal to 2 specific gas stations.
I usually fill up when I have 2 bars left or when I've gone my MPG times 10 (For example MPG = 48.5 therefore I fill up at 485 miles) whichever comes first. The technique for me is to insert the nozzle completely. Once the pump shuts off, I remove the pump and wait 20 - 30 seconds. I then insert the nozzle 1/4 to halfway and put the gas in as slowly as possible. Using this technique I can easily get my MPG times 10 with 2-3 bars left.
Now there's two things to watch out for this. First...not all pumps are good at shutting off when the gas is flowing slowly. So be careful of splashback. Second, some pumps' pricing algorithm gets thrown off when you pump slowly. So pay attention to how many gallons the pump says it put into your car.
FYI. On a roadtrip that put me through Boston, I filled up at a brand new Hess station. The nozzle was ported. I was able to fill up almost the whole way before the pump shut off. I certainly hope all future gas station nozzles will be ported.