Very important. Not only can I parrot the advice I have read, I can speak from experience. I bought a VFT last June and I put in a well-washed plastic pretzel container, filled with an inch of DI water. I kept the lid on at night and took it off during the day. the plant grew well. Each new trap was bigger than the previous, going from 1/2" -> 1". Then I had to go out of town for a few days. So I decided to leave the plant covered. The next several traps were smaller and or developed part way and blackened. As long as they sit in water, they don't even need a lid on for the night, either. Air circulation is an underrated part of the CP puzzle, just as important as appropriate light, water, temperature, food, and soil media.