1. You could run on reserve all the time, but you will be SOL when you run out because there isn't anything more after the reserve is used up.
2. The only difference leaving the petcock on whilst filling the tank is that there is some chance that the variations in pressure from the fuel pumping in might force some past the float valve and flood the carburettors. For that reason only, it is a good idea to shut it off before filling.
3. Fuel consumption can vary a lot, especially with a strong headwind (natural or because you are cranking hard on the throttle). Also, you will lose up to 3/4 gallon of the full capacity if you just stop pumping the first time the nozzle shuts off. The tank is officially full when the gas is fully up to the ring at the bottom of the filler neck. You can get a little more in by rocking the bike and getting some air out of the expansion area, but you should only do this if you are going to ride right away far enough to use up the extra fuel.
