Getting back to the original question, the problem of quotes within quotes and the confusion that can result is presumably at least part of the reason that CMS thinks it's fine to drop them for signs.
It's hard to read something with several sets of quotation marks, as here:
- Mary said, "I was going to open the door, but John stopped me: 'It says "Do Not Enter."'"
Eliminating the quotation marks for the sign definitely helps:
- Mary said, "I was going to open the door, but John stopped me: 'It says Do Not Enter.'"
(I'd rewrite both of these, but I'm using the clunky examples to make the point.)