Gary is right.
A registry cleaning utility could always be potentially dangerous.
Sure you could run one, remove some "un-necessary" keys perhaps and do no
harm...
but there is always the possibility of doing harm...so it's not worth the
risk.

Considering that there are thousands of registry entries.
(Maybe someone else could give an estimate)
it's not too likely removing a few keys that may no longer be needed would
actually do anything to improve your
system.

For definite registry problems...
it's best left to an expert who can do so manually.

FWIF: Since I am an experimenter (of sorts) and have expendable machines
available...
I have tried many registry cleaners and though they did not always seem to
hurt anything...
there were a numbers of times they did cause some minor damage.
I am sure that had a fooled around some more...I may have seen worse
results.

OTOH: After running a registry cleaner...there was certainly never any
improvement made
in the way the machine performed.

Such things as running anti-spyware utilites,
disk cleanup
and defrag generally do have noticable positve results.

(my 2 cents worth anyway)