Are holes neccassary?

I just built a rack and the gap between the top of the tub and the bottom of the next shelf is about 1/8", is it still neccassary to drill holes? I figure that gap will give a good amount of air flow.

I think that depends a lot on the atmosphere of the room the rack is in. Tubs with no holes might work well in a very dry atmosphere...but a room with even a moderate amount of humidity may cause too much moisture build-up in tubs with inadequate air flow.

The gap is plenty of air for the snake itself, but you may need to adjust for humidity levels.

This maybe a noobie question but what humidity is optimal in the tubs? 40%?

Im guessing its for ball pythons. If it is then 50-60% is the proper humidity. The holes arent only for regulating humidity, but heat also. A tub with no holes in the back to allow heat to come in can remain virtually uneffected by the heat tape in the back. My room is usually around 68-70 degrees and i have a rack from repti-racks with boot boxes in it. The back of the tubs are almost completely covered in holes to allow the heat in. I set the heat tape in the back at 107 to obtain the proper temps for what in the rack.

So since I am using back heat, 2-3 rows of holes in the back would be best?

i think that you should because it wont hurt the animal with some extra air