Samoa,
I agree with blonddocteur on this...when you are trying to match into something very competitive, sometimes the little things matter.
Peerie, if programs tell you they "want you to keep in contact" and/or would like to know that you are interested, vs. not, IMHO that is code for "If you want to be ranked to match, you'd better tell us you are ranking us #1 or very high". That's just my opinion, but it's based on what I've seen and experienced. Of course, I don't think it should make you go lie to every program where you interviewed and say you are #1, but I think you can assume if those type programs that said those things to you hear nothing much from you after you interview, they might not rank you very high because they'll assume you aren't interested. Also, IMHO this is a psychological technique used by some of the more mid-tier programs to try and get better applicants than they would otherwise. In my experience the really top places don't use these types of pressure tactics or try to court specific people as much, because they know they are going to fill with good applicants without playing such games.
I agree w/the attendings (who posted above) for the most part, though. In a lot of the lesser competitive fields like peds and IM, your telling a program how you are going to rank them is unlikely to change their ranking of you very much, if at all. They have such long rank lists anyway, and a lot of applicants get one of their top 2 or 3 places, so stuff like who wrote a thank you note or who said they were ranking a program #1 doesn't end up counting for much. Plus, like APD said, lots of the PD's might not believe you anyhow.