Okay. Let's talk about the elephant in the room. Dress codes. Not having them is a cool thing. Especially in a millennial context, to talk about your company as one that provides you freedom. The rise of companies like Facebook Google, Twitter etc., have projected this free and open culture to a point where it has become aspirational.
But is an enforced dress code really a bad thing? Well, the intention behind it was to avoid discrimination in the workplace, on the basis of religion, caste etc. If you think about it, the intention behind the enforced dress code was to promote equality in the workplace. The magnitude of difference in the way that people dressed would be much lesser if there were only 3 or 4 kinds of clothes to choose from.
However, over time, discrimination has seeped into dress codes as well. It's not enough that you adhere to a dress code - you must dress well also. And this isn't the late 90s I'm talking about. This is 2014. Take this recent article on LinkedIn which I read. It said something to the effect that you should take dressing up seriously, as it gives you a chance to rise up the corporate ladder, even if you aren't as meritorious as your compatriot who isn't the best dresser in the world.
I have a problem with that.
From being a cultural neutralizing agent in the workplace, the dress code has become one of those things that people resort to when they can't be excellent at something. In the same article, there were those who complained in the comments section, that people like Steve Jobs was popular, despite the lack of a professional dress code, and there were counter arguments that not everyone can be of the calibre of Steve Jobs, because of which, you'd need to follow a dress code.
Now, I can't speak for those who have spent years in the corporate world. They'd have surely seen the subtle ways in which things like your attire have made a difference in professional growth. And to an extent, I also agree that the dressing sense of an employee is a window into a company's ethos. But with respect top programmers, you'd be driving most of the good ones away by trying and enforcing a dress code.
I recently wrote about 4 things that will piss off your developer. The top of the list was a lack of freedom. Developers don't like restricting rules, and that extends to what they wear as well. Most of the developers that I've met so far, want to have the freedom to wear what they're comfortable in. And most of them are open to friendly advice if their dressing sense is offensive.
Keep it that way. Advice on what your developers should wear, and if they see the reason in it, they more often than not listen to it. Don't enforce it, for they will leave you.