I know you're eager to see how your mileage is--I sure was at the beginning! In any event, resist the urge to keep adding gas after the pump clicks off, unless you're SURE you are well under a "normal" fill-up amount. See
http://www.epa.gov/donttopoff/ for more info.
I'm eager to find out how the Altima Hybrid does, mileage-wise, so be sure you post after you have gone through 2 or three tanks.