I borrow Blaise Pascal's line of reasoning and look at this way:
If everyone sends one and so do you, then it's neither positive nor negative.
If everyone except you sends one, then it looks pretty bad for you.
If no one including you sends one, then it's neither positive nor negative.
If no one except you sends one, then it looks pretty good for you.
So all in all, your best bet is to go ahead and send a thank-you note. There's really no reason not to do so except for sheer laziness.