This is not required. If you only have a few ounces of fuel in a jug the air in the jug does not have enough water to be a problem. The most water you can expect to have is probably about .03 pounds of water per pound of dry air and at about 15 cubic feet per pound of dry air. The condensed moisture from less than a gallon of very humid air is only about 2 tenths of water. Since this is air that is over 90 degrees and 100% relative humidity this is just not enough moisture to worry about. How ever a loose cap exposes the fuel to an unlimited amount of air and moisture.