OK, the thing with the Chicago Manual of Style FAQs is this:

The CMS attempts to answer every question that could come up in copy editing, and it is resorted to in arguments between copy editors, of which there are many (arguments). But the latest edition, 15, which came out a year or so ago, deviated from the prescriptive approach of previous editions and got into this touchy-feely crap about how you're supposed to use your ear to decide. An example will help. This is from the Q&As on the CMS site:

_____________
Q. Section 8.167 (3) of the 15th Manual of Style says that, when applying headline style, a preposition should be capitalized if it is stressed (A River Runs Through It). Please clarify what is meant by “stressed.” Furthermore, how would you capitalize “One Nation under God”? Thank you.

A. We are talking about how it sounds to the ear—admittedly, a somewhat murky rule. A river runs through it? A river runs through it? No—a river runs through it. Your “under” is likewise a good candidate for capping, for the same reason. Other examples:
A Man about the House vs. All About Eve
Desire under the Elms vs. One Nation Under God
_____________

First of all, imagine the discussion that must have led to this question being asked. Second, imagine having your question picked to answer in this prestigious forum, and then getting this answer. There's nothing here you could take back to the person you were arguing with to say definitively that you're right or wrong. Picture the "discussion" that would lead from this: one copy editor saying "ALL about EVE" through clenched teeth while the other editor says "All ABOUT Eve" with equal feeling. There's no defensible logic, just conviction.

Non-editorial people follow this capitalization rule and other folk grammar notions when they write their own headlines and slogans. Verbs, for instance, should always be capitalized, but "is" almost never is, because it is rarely stressed: Shaeffer is the One Beer to Have When You're Having More Than One.