I did for a non rolling school that was my obvious number 1 choice. I followed it up with an actual letter of intent a little over a month before the decisions were made.
You can throw in a line about how your interview day solidified your desire to attend school X, or how you would be thrilled to attend school Y. etc etc etc. If at all possible, pinpoint something unique about the school that it prides itself in. But keep the thank you letter pretty short, and realize that its probably going to have little impact (these things are so generic, and to be quite honest a long letter in which you wax and wane will win you no favors). At best, it will get added to your file. At worst, it will go unread.