The rule of thumb to use in most interviewing situations, not just Software Engineering, is to dress one "degree" up from your interviewers. If you're being interviewed by jeans and a T-shirt, wear a dress shirt and slacks. If you're being interviewed by a dress shirt and slacks, you can't go wrong with a casual suit.

If you're being interviewed by a formal suit, though, don't go busting out the tux :-p. That's the top level.