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. On rare occasions I'll go to a third level page for really specific content; I haven't ever (to my recollection) gone to a fourth level.
When it comes time to wireframe each page, I might use letters to indicate different states or version of the same page.
I won't say this is the only way of doing things, but it hasn't faltered in years.