This is an interesting question about etiquette I just got from a student in my 12-week job offer intensive and thought to share it as well as my response. As you take this in, you can also assess what feels right for you. If you like this approach, I welcome you to use this.
The question is, “do I need to tell the interviewer that I'm sending them a thank you letter?” Do you need to “tell” the interviewer you’ll send such a note?
First, you have step one: you have the interview. Step two is to thank the interviewer. So, how do you want to thank them? Send the thank you letter, right? Or some sort of thank you. So the question is, “how should I execute my thank you? And do I need to tell them to expect the thank you?”
Let's just talk about “thank yous” for a minute, and then I'll answer the specific question.