i'm a big proponent of e-mailing people - and moreover, only people you really want to thank. nothing is lamer than a generic thank you note, seems reflexive and insincere in my opinion - but whatever works for you. e-mail is good cause its convenient for them, and they respond back (forcing them to think of you and a response), as opposed to chucking your letter in the trash after 2 seconds of reading it. i don't think it affects your app in anyway (i've gotten into places i simply haven't had time to send thankyous) - all the more reason to write selected, sincere notes(or e-mails). ofcourse people have their own opinions, and do what you may - this is just my take.
-jot