Some great answers already. One more thing to remember... If you are using aspen, or just about anything but newsprint, sometimes it can run up the humidity for a few days in a new set-up. If this is the case it will come right back down after a few days. Too much humidity is about the least of my worries. Just keep things clean and they love the humidity.

That said, I have much better luck with holes on the front and back of the tub and none on the sides. If you want to give this a try place warm side holes high across the back of the tub and cool side holes lower, about half way down the tub along the front. Also it helps to use more holes (about 20% or so) on the warm side than the cool side.