I think the fuel tank condensation and dirt arguments are WAY overrated. First of all, your tank is vented through a charcoal canister to a vacuum line, controlled by a solenoid. Little if any outside air ever enters in there. It's almost a completely closed system. Moisture should only be a problem if it's introduced by the gas pump.
Second, I'm not saying dirt will never get into your tank, but first, fuel is drawn from well above the bottom of the storage tanks when it's pumped into your car. There are also fuel filters at the gas pump that should trap any contaminants. I suspect that most peoples tanks are much cleaner than they would expect. Couple this with the fact that most manufacturers no longer include fuel filters on the maintenance schedule and I say hogwash to that argument.
My position is that you should drive it until the gauge is as low as you're comfortable with. I let mine get down to E, which gives me about 2.5 gallons left, subtracting the amount I pump from the tank capacity. In addition, I think if a few air bubbles get pumped into the fuel lines, it's not going to make a noticable difference. The O2 sensor will probably compensate for that condition by richening the mixture slightly to compensate for the extra air.
