I learned this at the interview of my first programmer job as a summer intern back in school. I wore full suit into an outdoor interview on a hot summer day. The interviewer laughed and said you don't have to be this formal, who happened to be the president of the development studio. The interview was short, which felt uncomfortable. I went back to dorm and changed into T-shirt and jeans, and then went back to meet him again to show some sample code I wrote. This apparently had impressed him and he decided to hire me.