I'm a hiring manager. Dress to fit in with the company, not your litter mates from engineering school.
When I hire, even when I hire technical types, one thing I'm looking at is how they would represent my company if they have client contact. I don't like to have to make excuses to my partners like, "Well, he's technical." Skills being equal, I default to social skills and dress.
Wear a jacket, at least. This isn't a frat party. It's a career. Time to grow up.