If you are running Windows 7 and are not having any problems, then don't run a registry cleaner. If you are having a problem, it's unlikely that a registry cleaner will make any difference.
If you are running Windows XP, then use registry cleaners at your own risk.
In all cases you should make a backup of the registry before cleaning. If you don't know how to backup and restore the registry, then don't run a registry cleaner. Registry cleaners often do more harm than good.
Check out this article for more information:
http://windowssecrets.com/top-story/dont-pay-for-software-you-dont-need-part-2/