Per Ken Blake:
removing a few hundred (at most) won't give you an
increase in performance. Registry cleaners can accidentally remove
important registry values, however, so there’s risk with little
reward."

To me, the ultimate registry cleaner is having an image of a known
"Good" system, keeping a log of changes, and re-imaging from the "Good"
image/applying changes if/when the situation warrants.

Implicit in that, of course, is the discipline of not keeping Data on
the System partition...
--
Pete Cresswell