I think a summary of the opinions I've heard here on this subject are:
1) It probably doesn't make any difference to PD's.
2) It can't hurt to communicate your honest enthusiasm for a program.
3) If you've come this far and invested this much time and effort in matching, you might sleep better thinking you've done all you can to get the residency you want.
From a ranking point of view, I'm looking for the best match and a big part of that is striking a chord personally with the faculty, PD etc at a program. In this respect I find it reassuring when I get positive followup from a program because I figure if they like me, there's a good chance it will be mutual. In the hopes that this may work both ways, I have written a "hello" to my favorite places, though I doubt it actually changes anything.
I guess it's good for your peace of mind if they write back and say something positive, and maybe that's the real reason people do it.
PS - a couple places did tell me that they like people who are interested to "keep in touch" and "let us know what you're thinking" because "we like to know that you're interested in us" though the majority didn't. I have to say though that I found someone asking you for love letters a real turn off.
