Now that is one thing I don't do. I don't do thank you e-mails. I feel it has kind of gone away and no one really expects them anymore, minus maybe some really true older timers. I just make sure to let them know I appreciate their time and their consideration for the opportunity after the interview. Maybe it's the field I'm in. I don't think it would hurt anything though but I'm not sure you need to continue to sell yourself on it, maybe just thank them for their time. I feel the digital age has really lowered the expectations for a thank you letter since people always have hundreds of e-mails waiting to be read anyway and snail mail is just not even considered. But like I said, I don't think it can hurt anything
The thank you email is not to thank them, it's to keep your name in front of them.