I'm not a PD, but I interview and attend our ranking meeting, and the PD is a close friend of mine.
Honestly--I totally discount 99%* of the post-interview correspondence. No matter how sincere you try to make it sound, it still comes across like the thank you letter you had to write to your great-aunt for the hand-knitted socks she gave you for your 10th birthday....by this I mean a very unappetizing combination of obligatory and obsequious.
At our program your initial ranking is determined by interviewer ratings filled out on the day of the interview. You might edge up or down based on some intangible we discuss at the meeting, but frankly you can tell us you've made us #1, and even if we believe you, it's not going to put you ahead of someone we might want more on our list. It's also unlikely to make much difference in comparing you to similarly rated peers--you're either in the Top 10, the top third, or "OK". If you're not OK--we won't rank you. (Also, I have seen candidates where we thought they were "mostly OK" make it easier for us to drop them off our list entirely by being inappropriate or unintelligible in their post-interview contacts. So really, anything more than "thank you" is potentially asking for trouble...)
*What's the 1% you ask? It's the rare occasion an applicant has made a real impression, a personal connection, and is following up on it--and that's rare indeed in a 30 minute interview... But if it's not truly there, don't try to fake it!