It is best to send the thank you letter before the interview.
- Receiving an Interview
- Sending a Response for an Interview
- Sending a Thank You after the Interview
You may also send a secondary thank you note once the interview has been completed. This again shows professionalism that is above others that were interviewed or at least on the same level. This secondary email should be short and just remind them that you were interviewed. If you received the job during the interview you can also include a thank you for being hired to show that you are excited and ready to work for the company.
It is just a more professional way of handling interviews. Even if you do not get the job send a thank you after the interview.