Around ten years ago, I worked for IBM as a software engineer. The dress code at work was pretty casual, so I tended to wear jeans and a t-shirt in the office. One day, I was asked to speak at an event that would be attended by a lot of our customers. I asked my mentor, "Should I dress up? Should I wear my suit for the first time in years?" His response surprised me. He said, "These customers have come here to talk to engineers. Do you want them to think we've sent a salesman to talk to them." He made it perfectly clear that I should dress like an engineer. I confess I did get rid of the t-shirt; I wore a casual shirt with my jeans that day, and the folks that I talked to went away satisfied that they had indeed spoken with an engineer.