if your fourth year is sooo boring that you have nothing else better to do go for it. Others have such high sphincter tone that they do personalized letters to each person they interviewed with. Namy programs i went to flat out said, "dont send thank you notes, we wont read them". Then the REALLY paranoid applicants would be chattering later in the interview day "is he just testing us with the dont send a letter thing?". Bottom line, if you feel like you have to to soooth your own nagging compulsions, go ahead and send them to whoever you wish. Like the people here are saying, it has been documented that it really makes no difference (recent JBJS) I dont mean to come across as rude, if i did i am sorry, but it got SOOOO old by the end of the interview trail listening to people worry about things like this that i wanted to scream