Wow this is an old post. CCleaner's registry cleaner is pretty safe, but like most say, there isn't often much reason to use one. However, it is a pretty good way of getting rid of references to software/installers that no longer exist and other little things. But you won't get a noticeable performance boost from it.
I personally use SlimCleaner. IMO it's a much better, faster system cleaner. It cleans more than CCleaner does, and it does it faster. And before anybody says it, no it doesn't clean anything unsafe. I've been using it for 3 years. That being said it is a powerful cleaner. You need to customize the cleaning options or you may remove custom start menu orders or other stuff like that if you don't pay attention. It will blast what you tell it to. I use it probably once every 2 weeks or just whenever I notice things starting to run a little slow. If you're a 1337 web surfer like me, you can probably clean gigs of junk data in just 2 weeks time. I think the most I've ever removed was ~12 gigs of junk but I think that was in a months time.