Clients ask me what's the best way to send post-interview thank you notes - email versus snail mail. I advise the latter, sending hand-written notes. Email can be viewed as less labor-intensive or thoughtful.
You can still get the notes in quickly: Put them in the mail the morning after you've completed your interview. Some applicants even bring blank thank you notes to the interview day, complete the cards after their interviews, and leave them with the program coordinator.
Your thank you notes should be written on plain cards and sent to every faculty member you conversed with one-on-one. If the residency coordinator helped you with a difficult scheduling issue, for example, writing to him/her would be wise too. Within reason and if written cordially, a thank you note cannot hurt you (unless you have been expressly asked not to communicate after the interview day).
Consider them low-hanging fruit.