I've only ever used ccleaner for its registry cleaner, and only when I wanted it to clean something like an incomplete uninstall that I suspected was causing a problem. Never had it run "in the background" for anything.
CCleaner's Smart Cleaning feature runs as a background process (if it's enabled).
I can't think of any reason why you shouldn't just go ahead and disable it if you don't want this feature. I wouldn't want it running in the background either.
I've never had it running all the time either but realised it WAS doing so after the last Ccleaner update - and went to disable it. But got a dire "warning" when I did. So I came here for the darn good advice I knew I would get !!