Most carnivorous plants require highly humid conditions.
But since most plants desire some air circulation, I do not recommend you seal the terrarium. Air circulation seems to be particularly important for Pinguicula native to the U.S., while Nepenthes do very well in sealed terraria. Some people do not use terraria, but instead put their plants on a plate or tray filled with water or pebbles and water. This may help the humidity a tiny bit, but I do not think it is very effective. If you live in a humid area, this is probably not required.