If your interviewer is wearing jeans, a t-shirt, and sneakers, then you should wear slacks, a button down shirt, and leather shoes. If your interviewer is dressing business casual (slacks & button down), then you should wear a suit. I currently work in the least formal corporate environment that you could imagine, but I still wore a button down shirt (tucked in) with jeans and leather shoes. I now wear t-shirts and jeans/shorts to work every day.