I would drill holes as well. There's enough air to breathe with 1/4" along the top, but almost zero air circulation down by the floor that way. I would be worried about making petri dishes out of the tubs. I've seen a lot of people who don't drill holes and the tub walls are always fogged up and wet. If the animal pees, there is uncirculating ammonia air in there. The list goes on... if you need humidity, I would always run a humidifier in the room before sacrificing air flow for it.