Please and thank you goes a long way during the interview process. It starts with the moment you open the door. If you don't think the front desk person carries any weight in the company you are dead wrong. They set the tone for the interview. If you are rude and disrespectful when you walk in, guaranteed interviewer will hear about your attitude when the front desk lets them know you are waiting. So before you even meet your potential employer, their guard is up, a negative tone is set and you will be in for a much tougher interview. The Wall Street Journal corroborates this in the article "Interview Etiquette Begins the Minute You Walk in the Door", http://goo.gl/jbT782.
Be smart, set a positive tone. Employers want to hire people that they like. So make yourself likeable. A friendly and courteous demeanor goes a long way and coupled with the right skills and work ethic can even land you the job.