Well, there is this about Brent Wagner “He once spent an entire hour and a half lecture on the importance of thank you cards” http://www.detroitnews.com/story/entertainment/2016/05/09/michigan-musical-theater-head-retires/84159764/
And then there’s this article on Rachel Hoffman and thank you notes.
http://www.music.umich.edu/muse/2010/fall/spotlight-hoffman.html
I completely agree with @vvnstar, and every audition masterclass (CCM, CMU, OCU, NYU, TSU, Michigan, Pace, and Syracuse are just a few that come to mind) my kid attended mentioned the importance of thank you notes for college auditions and beyond. Michigan felt so strongly about thank you notes that the Mpulse kids were sent an email upon returning home that had names and emails of everyone they had been in contact with throughout their time there (including admissions personnel) so that they could send each person a personal note. And, those kids also paid for the opportunity to attend that program. Thank you notes are just nice.