You should leave it on because you need the air in your cab to be constantly replaced. This ensures a fresh air supply for your girls to breath and freshens the cab somewhat until the next light cycle comes on which helps keep the temps at bay IMO. Stagnant air can cause slower respiration rates and thus reduce photosynthesis which would cause slower growth. Not only in your cab but more specifically around the leaves the air should be replaced with fresh air to ensure proper respiration and transpiration.