I have debated this and its been debated here, MY companies policy is that anytime we have to force air into a wall cavity we set them, this being because we really don't know what is inside that wall, 75% of homes 20 yrs old or more have mold somewhere, this may not be able to be seen, there may also be animal feces or who knows what in the wall cavities. or of course if its cat 3 water or some other special situation.