Culture is aways important. I am baffled as to how someone could even remotely suggest this is not important, other than as Jerome stated (perhaps a little harshly :)) has no experience running a business. Stating developing a culture is not important...ESPECIALLY when you are small and starting...yeh..ok..Im with Jerome on this one, probably some of the worst advice I have ever heard. Delivering a good product is not culture, it is a result of what the culture builds. Kepping customers happy is not culture, it is a result of a great culture that has values that support customer satisfaction. Definitely some confusion there.
From the minute the idea to build a company pops into your head, culture is a key element in the foundation of your business. But before that it is a key element in even getting to each new day. To Laura's point, farrrrrrrr too many people limit culture to what kind of furniture is in your office. It is part of it but very small in comparison to the mentalities and characters and values within the leadership and staff. I always find it hilarious that so many start ups I see seem to tout having a pingpong table, or the company dog. Seriously look up tech start ups in Austin or other trendy cities. I would say at least 1 out 4 have a profile of a dog.
The damn dog is not culture! Having people who are caring enough to clean up dog mess might play into that a little I suppose!
The people make culture as has been stated already, and the people impact it. A recent job posting for a successful company I was dealing with bragged, "The culture is great. All the free snacks you want." I commented to the CEO, "Do you really think a smart employee evaluating financial planning for a household is moved by bulk snacks you buy at Sams club or Costco? Do you think you will attract top talent with snacks that cost a quarter?" That is a smoke and mirrors presentation of culture, as is "cool." As if eating free snacks encourages employees to work harder for greedy or manipulative bosses. Knowing that you can not shower for several days or wear deodorant does not make your culture relaxed...and it likely annoys those with personal hygiene.
Be yourself. If you are a jerk and lack integrity, people are going to see through it regardless of facades. If you have character, which hopefully you do, let that be the example that shines through in all decisions and actions in your business. As you attract the right people who posses valuable qualities that truly influence culture, then you can worry about how free snacks or a ping pong table, or dress code impacts culture.
Culture is absolutely important for you as a startup, because you need to know what type of people you want to work with and for you.
To finally get off my soap box. I want to say one more thing that is far more important than ANY of the advice given on culture. If you are barely finding time to spend with your family, that is your first priority over defining culture or any other aspect of your business. A lot of Alpha male (and female) business types brag about not having time. I recognize you were not saying this trying to be cool like most, but rather seek advice. There is nothing admirable or "cool" about not having time. Whenever I hear entrepreneurs brag about this, I think "You are a poor manager, because you can not prioritize and delegate in a way that does not create chaos in your life." There is nothing more entrepreneurial than having a family period. And there is no greater entrepreneurial success than building a strong family. Im sure you know this, but in the midst of start up chaos, I pray this always takes priority.