Is a follow up letter after an interview ever a bad idea?

No. The follow up letter after an interview is never a bad idea. If your job interview went well, sending a follow up letter after the interview will only serve to emphasize your enthusiasm for the position as well as your knowledge of business etiquette. If your job interview didn't go well, the follow up letter after an interview is unlikely to do any harm. In fact, you can use the opportunity to answer the questions you feel you didn't fully answer during the interview and clearly state your experiences that make you qualified for the position.