Oddly enough, in stuffy industries not adhering to the dress code is a strong social signal that you're good enough to be able to flaunt it.

I used to work in telecoms consulting, which is fairly buttoned up: usually dress shirts, slacks and nice shoes, although us engineers were permitted to dispose of the tie and open the top button. But the best engineer in my company -- the guy who'd be flown in by private jet when needed because the billing systems were dropping $1m/hour on the floor -- maintained a steady uniform of ripped jeans, ratty T-shirts and permanent 3-day stubble, and the clients would always fall over themselves with gratefulness when he showed up. Not because of what he wore, of course, but because he was "the real deal" and very good at actually fixing problems, instead of talking about them.