Numbers under ten should definitely be spelled out, but what about the case where your use of them indicates how they exist in the thing you are describing? For example: “I stepped into the elevator and pressed the button marked with a 3.” Should that be three? Because the number isn’t spelled out on the elevator button. Moreover, if the answer is that it should be 3, should the 3 be in single quotes?
(I realize there is a more elegant way to construct this sentence, but my question still stands.)