Registry cleaner in CCleaner is very mild and will find keys which are usually safe to clean, but I will not clean anything I am not sure about.
But that's not a problem, problem is that you don't know what CCleaner do. The standard file cleaner actually deletes the registry keys, and will not show a thing. For example, open something in WinRAR and you will have history of open files, also you will get registry key (folder) called ArcHistory under HKEY_CURRENT_USER\Sofware\WinRAR. Now open CCleaner and right click on WinRAR and clean. Now go to registry again and you will not find anything. Also with 7-Zip, in that case it will delete FolderHistory and PanelPath (under HKEY_CURRENT_USER\Software\7-Zip\FM).
Keys usually only stores history informations.
Also, if you look at WinApp2.ini file you will find that CCleaner have numerous orders to clean registry keys. Very often it will use registry for detecting programs but under default value (which is usually false), it will show what will eventually clean. Search for RegKey, there is more than 1000 matches in Firefox (I used Firefox for quick search of raw content of WinApp2.ini on GitHub). CCleaner will only show found or cleaned files, not keys.
If possible, I demand an answer from developers.