I disagree. The recruiting process is still based pretty much entirely on human judgement. If you come into a t-shirt culture wearing a suit, it hurts you. The interviewer is looking at you thinking "why is this guy in a suit?" and it's a strike against you, just as surely as coming underdressed would be. You don't want to give the interviewer superficial reasons to vote "no hire".
> I'd call up the firms you've interviewed for and ask what they liked and didn't like about you, ask them to be honest. Most interviewers will answer!
No, they won't. Most large companies have policies against revealing why a candidate was eliminated. Revealing that info opens them up to litigation. "Oh, you say I wasn't hired because I don't know Python, but then you hired this other guy who doesn't know Python. Must be because I'm [some trait that probably wasn't a factor but might be grounds for a lawsuit if I can convince 12 random people on a jury]".
Google, the better I-banks, etc. will have a recruiting process that's professional enough that they won't reject applicants because they were a bit too formally (or informally) dressed -- especially for a position that won't be customer-facing (I'm assuming you aren't applying to sales jobs).
I'd call up the firms you've interviewed for and ask what they liked and didn't like about you, ask them to be honest. Most interviewers will answer!