Well thats a difficult question to answer Grandpaw.
Bob Proffitt was quite right when he said the reg-cleaning has not been such an issue with Windows XP than it was with Windows 95, 98 or ME. With those systems the registry was always very fragile, and could fall over, (crash the system), at the slightest nudge.
Windows XP is much more robust, and I am sure there are many people out there who have never even heard of registry cleaners, but do not seem to have many problems, (apart from viruses, trojans, spyware, Home-Page hacking, worms,......... ).
A lot depends on how you use your machine. If you install a lot of programs, then uninstall them and install others, and so on and so forth, orphan regisrty entries will eventually build up, and as the Windows OS has to work with the registry all the time, there may, after a while, be a slowdown in performance. But then processors are so fast nowadays that any degradation in speed will be negligable.
The fault with orphan registry entires rests with 3rd party programmers. When they develop a program to be installed, they place registry entries all over the place, and very often do not include sufficient information in their uninstall programs to delete them all. Also, programs install files all over windows, especially in the Windows/System, (or Windows/System32 folder), rather than keeping everything in their own program folder, so there are often orphan files spread about Windows folders as well. In addition, if any programs allow users to store data, eg saved games, or documents, templates, etc, these will be left behind after an uninstall.
In my Windows/System32 folder I now have 2032 files, and I started with 1200 or so this time last year when I purchased the computer.
Windows XP does its best with its Uninstall files, but something usually gets left behind.
So, Registry Cleaners do a good job in that they help to keep the registry clean of clutter. Some better than others, and some are easier to use than others, but like Bob says there is not the powerful need for them that there was with previous Windows OS's.
I do use a registry cleaner, but not often. I don't install or uninstall that many programs, so I will only run my cleaner occasionally, once a month, or once every two months or so. Often it will sit there for a few months, unused, and unwanted, 
Mark