I always use 0.0 for the Home page. If there is another state of the home page (logged in, or admin user, etc.) I might use 0.1 or 0.2. Sometimes I will also give a global page a 0.x number, like Search Results, since this doesn't belong to any content category.
x.0 maps to a category page. Not all category pages have children.
x.x maps to a child page.
Most often, I will only go to a third level if there is a process embedded in a child page, like submitting a form or completing a purchase.
