You guys know what I do with all my thank you letters/emails?
Now, don't be hurt when you read this. I'm just being real.
When I get a thank you letter/email--I quickly skim through the whole thing looking for any question marks or anything that stands out. If I see none, it goes to the deleted items or trash. Seeing question marks in thank you letters actually annoys interviewers, because they've just spent a whole day asking and answering questions. Do you think they want to spend more time doing this?
Investment Bankers work 100+ hours a week. They don't have time to sit there and read through a thank you email from someone who probably got dinged during the interview process and reply back. It's a waste of everyone's time.
You'll know the interview went well if you get a call/e-mail back from HR later that night with an offer or an invitation to proceed to the next round of interviews. Sending a thank you note/email isn't going to make or break you. The reason why is because the people who interviewed you already knows who they want to bring back or give offers to a few hours after the interview. By the time your interviewer gets your thank you note/email--you probably already got the offer/invite to next round or a rejection email from HR.
Most people in my current office and my former office all share similar practices. Again, this is the culture in finance. It may be different in another industry.