My advice for the average computer user is DO NOT using a registry cleaner. If you do not have knowledge of the registry, then you are far better off leaving it alone, and definitely not placing blind trust in a program to do the job for you.
Win 7 does not need a registry cleaner. Forget all the "wisdom" you learned about XP. Win 7 is not XP and does not manage the registry the same as XP.
Win 7 is much more efficient at managing the registry than previous Windows versions. If you are very knowledgeable of the registry, you can use Ccleaner to delete keys left over when uninstalling programs. However, these few keys will not make 1 millisecond's difference in performance. If you run Ccleaner or any other registry cleaner and do not know precisely what you are doing, you will have problems down the road. There are no gains to be had from using a registry cleaner and the risk is great.