I clean my throttle body often. Of the many times I have cleaned it, I have only taken it off once. Yes, you can do a better job with it off, but I don't think the slight difference is enough to matter. I do it with the engine off. Cleaning it with the engine running will not do as good of a job, but it does something else in that it tends to somewhat clean the intake manifold and combustion chambers as well. I agree that cleaning it with the engine running is probably rough on the cats.
Just another opinion.
