Matt & Catman have it right. Everywhere there is condensation, sunshine or rain, ventlate. Ventlating the hulls makes sence year round. Many sailors will crack the hull covers when they get on the beach to reduce pressures from the heated air in the hull.

I've seen big boat sailors shrink wrap their boat in the fall and not have openings for ventilation. In the spring they have many gallons of water in the bilge. Those that ventilate have no water or mold problems. The PVC in a extra port cover is a good idea.