Is it important to send a thank you email or letter after a face-to-face or telephone interview ?

Having been on both sides of the interview process, I believe the "thank you" letter or email is just one more piece of information about the candidate's level of performance and principles.

I believe a follow-up "thank you" email or letter goes a long way especially after a face-to-face interview. If you are writing them make sure you write one to each person to spoke with and note just the hiring manager. In the note mention what you all discussed or something that will help you stand out in their mind. Also make sure to indicate your excitement for the opportunity you interviewed for and how your talents can help/improve their organization, department, and/or team.

I'm probably old school, but I say yes. If an individual has taken his/her time to meet with you they deserve to be thanked, if for nothing else, for their time. I always used small "Thank You" cards with a blank interior where I could be specific on something from the interview. Small size of card also helps get past mail room or gate keepers who will think its personal and not open/screen.

It's also a great opportunity to deliver your brief "elevator pitch" to reinforce your image

It's always been my practice to send a hand written note for any situation that requires a thank you, COL Mikel J. Burroughs.

Every recruiter I've talked to, and every resource I've used on my way out of Active Duty has mentioned 'Thank you' notes as being good practice. It can't hurt, it can only help, it is not a big investment of time or resources, but the return on investment could change the path of your life. My perspective is this, take advantage of every opportunity to give yourself a leg up and to make yourself stand out in a positive way. This is one of those ways.

Common courtesy goes a long way.

In DoD contracting networking is everything. After that it's probably people skills, by that I mean how well can you integrate into a culture. Can you "get along" with pretty much everyone on a professional level or at least give the impression you can. As for follow ups, If I was really interested in the position I always sent a follow up email to the interviewers even if it was an over the phone interview.
I'd make it a point to highlight whatever portion of the interview I felt was the best part, even if it wasn't job related. I did this because I figured the interviewers already knew who they had in mind to hire. If I was in the running I wanted to stand out individually so I took advantage of any high points that occurred during our time together I'd try to spin it in such away to make it both personal and professional.

COL Mikel J. Burroughs - Absolutely after face-to-face interview. If you want job otherwise. Sandy :)

Viewpoint: corporate project manager making hire recommendations to corporate officers, Army Civilian manager, conducted hundreds of interviews for jobs in $50K to $90K salary range.

Yes, send a thank you e-mail or letter (rare these days) to the people who interviewed you. Also, to the receptionist or secretary who may have greeted you, shown you courtesy, and helped you find your interviewer's offices. The thank you e-mail does two things: it puts your name back in front of the people making the hiring recommendation or decision; it shows you have good manners and basically give a flip.