It is never a bad idea to follow up after an interview, either by email or formal letter. This shows you appreciate being considered for the position, that you are still interested, and that you are waiting for a response. How to write a follow-up letter that will keep the lines of communication open, who to send the letter to, and when to send it are all covered in this podcast.
A post-interview follow-up letter will show your potential employer that you are grateful for the interview opportunity, as well as committed to the position. A follow-up letter will also give you one last opportunity to make an impression.