I think it really varies by program. Places like UNC that get 450 apps and interview 100 people don't have a need to put 100 people on a rank list. Other places that are less competitive might rank nearly everybody they interview in order to fill. I had heard an estimate of 2/3 interviewed were ranked, but I don't know if that's a set # or just what you get when you cross out the bozo factors and the "I'm not working with that person" factors or if the number is even correct, since I can't remember the source.