There are two different schools of thought on this question, and I believe that there is no right answer. Instead, I believe each candidate should do what makes him/her most comfortable. While some believe that a thank you note is absolutely necessary, I believe sending a thank you note could actually hurt your candidacy if it is not done correctly. In my personal experience as a member of the hiring committee at my old firm and a frequent interviewer of lateral candidates, a thank you note never changed my mind regarding a potential candidate. My decision was always based upon my personal interaction with a candidate. At most, a well-written thank you note from a candidate whom I had liked simply reaffirmed my positive impression. However, it never changed a ''no'' decision to a ''yes'' decision. Thus, I do not believe that thank you notes are absolutely necessary. However, I am sure that there are many people who would disagree with me. So, if you are going to send a thank you note, there are several important rules you must follow.
''Thank You Notes: Are They Still Necessary?''

First and most important, make sure that there are no typographical or grammatical errors in your thank you note. If necessary, have someone else (like your recruiter) read your thank you note to make sure that it does not contain any embarrassing mistakes. This is your last impression with an interviewer, and you want to make sure that you do make the most of it. For example, one of my colleague's candidates met with two different firms on the same day, and was in a rush to send out his thank you notes when he got home that evening. Thus, he typed out a simple, generic note and ''cut and paste'' the same thank you note to each interviewer. Unfortunately, he forgot to change the name of the firm. Needless to say, this made a bad impression on the firm and the candidate did not get the job offer. It is impossible to say whether this one mistake was the reason for the firm's decision, but it certainly did not help.