On gasoline tanks, I found that having the vent line at the interior top of the tank with it exiting the tank and then the model at a point lower than the bottom of the tank works fine. That way, if inverted, no fuel will leak from the tank yet the tank is free to vent to the atmosphere. Some use two vent lines, one as above with the second one being at the bottom interior of the tank and exiting the model at a point higher than the top of the tank. Again, if either upright or inverted, no fuel leakage yet open to atmospheric pressure. I've found no advantage in having two vent lines.