We use a 2:1 Vermiculite/Perlite blend. I used to always open up the egg box once a week, mostly to check the eggs, but they obviously got some fresh air while the top was off. Toward the end of last season, we were so busy with hatchlings, that I didn't have time to check the eggs that often, so many of our last clutches went a month, and some the entire 60 days without ever getting any fresh air. I did not notice any problems from doing it either way. As Rapture stated, I think the only regulating force here is checking too often, and having the humidity stay too low in the box. As long as your heat is spot on, and your humidity stays very high, I don't believe they need any "fresh air".

Hope that helps,