There needs to be some ventilation under the valve cover, so either leave them or close them up and use a vented oil fill cap. One issue with the back vent opening is the drain tube from the cowl cneter vent. If the hose points directly down to the center, there's a good chance that water can get into the valve cover. The tube is supposed to be offset to one side, but they can get limp and point toward center. If you leave them open, definitely check that drain tube for correct positioning.
