I would insist on storing a Sunfish with the drain down. Hollow vessels of all types draw moisture inside, where it collects at its lowest level. It's temperature changes and moisture that bring this "defect".
Some Sunfish clones have drains on the opposite side, and some manufacturers consider it important enough to install a pair of drains. My Porpoise II clonefish had (what I think was) a chrome-plated brass vent in the transom.
A big consideration is to open the drain when storing on edge.