Unless they are bearding on the front, I would leave them closed up. Since they are just getting started, I wouldn't hinder them by letting their heat get away. They need it warm to build comb and raise brood. By opening up the top, you are giving them a draft thru the hive. New bees won't be born yet, so the population is continuing to dwindle until the brood hatches. So with fewer bees, it becomes harder to keep the hive warm.
rob...
oh, if you can just put a small block between the inner cover and top cover to ventilate. If you prop the inner cover, they will build burr comb on the top of the frames.