I have never seen .. either personally or in digital/photographic/television displays .. dreadlocks that looked clean. So I would have to say that I agree with your final statement ...
Actually I just read up on washing dreadlocks and they are washed with special shampoos while tied up in dreadlocks. This will only get them half clean (my opinion) so I guess a few times a year the hair is fully undone and fully washed.