Always dress up and dress conservatively for a job interview. Even if you are applying for a job behind a steam table in a fast-food restaurant or in the relaxed environment of an IT company, this is a strategy that works. Remember how your parents told you to dress up to go to church or to visit Grandma? Their reasoning was that we show respect for an organization or an individual by dressing up. Your polish indicates that you think the interview and potential employer matter and that you respect them and the situation.