In terms of the pre-interview one -- I just think it can put your folder back on the top of the pile. And sometime schools specifically want to know that you are interested in them, so they'll wait to hear that you are before they extend an invitation. (I'm thinking specifically of Wake, Cornell, and a few others.)
I think this strategy is more effective post-interview, though. I would try to write the letter to a specific person, rather than the admissions office in general. In order to make sure that it gets into your file, it should go to a specific person that is on the admissions committee. In my experience, I have generally met with one or two people at each interview who are on the admissions commitee -- either they interviewed you, or you had a brief discussion with them, or they introduced themselves to the whole group at the beginning of the day.
In the letter, you could remind them about your meeting with them, your discussion or whatever. I usually will restate the things I told them at the time. For example, "as we discussed on __(date)____, I am very impressed with x, y, and z. I believe this medical school is a particularly good fit for me for my education because ____." I always try to include an update in the letter, because I think it's a good segue and it makes the letter seem not quite as overtly for sucking up purposes.
I have done this to the schools I really care about, and every time the person has responded to me and said they appreciate the letter and my interest in the school and they would make sure it would be included in my file. And I just got accepted at one of them!!!!
I don't think it matters if you do mail or email. If you send an email, they will be more likely to respond, although I did send one via mail, and the guy actually took the time to look up my email address and respond to me. I was pretty impressed by that.
I would do this in addition to the thank you letters. Try to space it so enough time has passed after the interview that you have something additional to say -- if not an update, at least you can say, basically, "after all this time has passed and I have had the opportunity to see other medical programs, I still remain strongly interested in your program." But you do want to make sure this is done in time for it to get into your file before your file goes to committee. You need to be fresh in their minds when they begin to discuss you at the meeting. I truly think this helps.
