Cleanse Tag reduces the chances of a wild Pokemon appearing (but only if it is a lower level than your lead).
According to Bulbapedia, the chances are lowered by 1/3 in Gen. IV. The reduction in the encounter rate is unknown in the other generations, but it is likely that it'll be the same.
Unlike what most sources (ex: Serebii, Pokemon Wiki) imply, they do not completely get rid of wild encounters.
I probably could help you test this out.. but for now, I hope I helped!
Source: Experience, Bulbapedia Link, GameFAQs / GameFAQs #2