When is it appropriate to wear Business Professional to a job interview?

Quick story: I was a nurse case manager for the WTB at Walter Reed Army Medical Center, and then transitioned to Fort Belvoir, VA. I had a Warrior in Transition who was from West Virginia, and I found out about a Jobs conference in Springfield, VA. Knowing that he needed some help, I asked about his resume, which he indicated that he was good to go. I then had him go buy a suit (informed him that the salesman would help him with the tie, shoes, and socks). Unfortunately, the day came and I was out of the office. He was frantic, and my partner in crime helped him tie his tie (this was the first time he had ever worn a suit). He went to the conference and came back to me and informed me that he was one of only three people who showed up in a suit and tie. By the way, he is now working for $70,000 in the National Institute of Standards and Technology.
BLUF: Want the job? Dress the part.

It is always recommended to dress professionally, most of time you can't go wrong being overly dressed. But it does depend on the industry as well. With that said, you are never wrong to dress nice, dress slacks and button down collared shirt at a minimum. If you are going to any kind of professional job always wear a suit and tie (or for women the appropriate equivalent).

I agree with everyone who's responded (just figured I'd add one more). When you go on the interview they'll be looking at you as a representative of their company. You can't go wrong in a suit and tie.