Somewhere in the book written by the founders of 37signals they allude to the premise that we should all remember that the term "start-up" is interchangeable with the term "business". As such, you have to protect your interests as if you are running a business, and not get lost in the idea of a start-up. Getting things in writing can be tricky prior to launch and proof of persistency, but at the point where you are hiring additional staff and opening satellite offices, the topic has to be addressed.