If you write that you should just be very specific about why you will rank them highly. For all we know you write that in every thank you note. In general I would say writing about where you will rank a program is not super helpful. Simply express your enthusiasm about that particular program in some form, preferably during the interview. Nothing worse than interviewing someone who has no ties to the area, has no particular interest in the city, knows nothing about what your program's strengths are and just seems to be going through the motions because they got an interview.
Some people, after they've interviewed everywhere will write a note to the program director at their top choice to let them know their intentions. I've never been a program director so can't tell you how much that helps, but I would say it can't hurt.
Doesn't matter if you got the interview just a few days before due to cancellation. As an interviewer I am never made aware of that info.