I have always been very careful when using regisrty cleaners, I have often had to fix machines where registry keys have been deleted that are actually needed and results in system instability and BSODs.
I'm not saying don't use them, just make sure you have registry backups before you do anything. I know CCleaner is supposed to do this by defualt, but I have encountered corrupted backups and other issues that have caused no end of problems.
Burnsy