p's and q's
It's safe to say that I have fairly strong feelings about thank-you notes. Namely, I view people who don't write them promptly as bad members of the species, raised by wolves in an uncouth forest full of those who lick their knives at formal dinners and men who let their dates walk on the street side of the sidewalk. And, try as I may, I almost never forget who failed to send thank-you notes after his/her wedding, baby, birthday, etc. (February 2005, I'm talking to you.).
Love keeps no records of wrong, but Miss Manners certainly does.
So normally I would say that there's never a reason to not write a thank-you note.
(A real one. On stationery. Email doesn't count.)
Unless, that is, you're a politician accepting huge gifts from lobbyists and other donors and then failing to report those gifts on your Senate financial forms.
That's the one and only time I can think of that thank-you notes might be a tiny negative.
Especially if it's used as evidence against you in court.