Over time, the Windows Registry can begin to contain information that's no longer valid ... Eventually this orphaned or misplaced information accumulates and begins to clog your registry, potentially slowing down your PC and causing error messages and system crashes ... Cleaning your registry once a month via the Windows Live OneCare safety scanner is the easiest way to help avoid these common problems.
Microsoft Live OneCare Safety Scanner > about the Vista/7 edition of the scanner > what is the registry/why should I clean it?
What's new - Windows Live OneCare safety scanner for Windows Vista and Windows 7
As others have said, using a registry cleaner is a personal choice. Microsoft recommends it and provides its own FREE cleaner. As with everything else computer related, if you're going to clean/edit/modify/change/erase/delete/alter/etc anything on your machine, it's not all that hard to backup or image the PC before tinkering around.