I store mine closed but barely. I spin the vent until it makes firm contact, then back off and reset again to the point when I feel contact and stop. This way it can vent at a low pressure since we have varying temperatures in sunny...no rainy (Today) California. My fuel tank has a gas gauge and the nipple on it, which then requires a gasket around the unit, which is held down with screws. If the tank is sealed too much the gasket will be the first thing to fail.
One of the keys is any vessel/vehicle stored for extended time periods needs to be store with a FULL FUEL TANK! This leaves a small air space in the tank, so less expansion and contraction that can force the fuel vapors out, and moisture in, which is really bad.