It's great to define organizational culture, but let's keep in mind that it is bound to be very fluid in a new startup. When you're very small each new person will have a pretty significant effect on the culture of your organization (of course the marginal effect of each new worker would be diminishing). Over-defining it at the beginning could be a mistake in so creativity-intensive industries like hacking.

I'd actually take an approach of formally defining the culture once it has established itself (and is working well).

But well, there's still too many variables to consider. Personal culture of the founder(s) would be very important in picking the right approach. Although many people would like it to be，I don't see management becoming a hard science anytime soon.