some people drill holes, some people cut it out. whichever works for you but either way helps with cooling. i personally like the look of a nice arranged grid of holes. its not as needed on regular truck bodies, since the head sticks out of the bed. the suv bodies, and the jconcepts, bodies, or anything that the head stays down in the body some, may need some help.
as far as "breathing" as in more air in.. not so much. the cylinder on these is about the size of a thimble. it doesn't take a whole lot of air to keep moving. as long as you have a good, clean filter, your gravy.