For a recent commercial audition, I was asked to read for a character called 'diner waitress'. I wore jeans, comfy shoes, and a solid colored plain (no logo) knit top. At the audition, I saw women sporting aprons and carrying order pads.
When I auditioned for a scientific expert character, I wore a suit, and watched others walk into the audition wearing white lab coats.
What gives? To me this behavior is gimmicky. It's called acting. In terms of wardrobe for auditions, I always do my best to give a suggestion of the character with clothes that are in my closet. After seeing this extreme costuming behavior at several auditions, I checked in w/actor friends, and it was almost universally agreed that dressing to give the suggestion of character is the way to go.
This coming Monday, I'm auditioning for a drill sergeant character. Instructions from my agent were to 'dress the part'. When I queried further (they don't really want me to show up in fatigues, do they?), I was told boots, khakis, and a plain top. I think I'm going to bring my mirrored sunglasses as well, in case they want to see them. I'm curious to see what others wear to the audition...