But when push comes to shove, "should" has this definition: Should: verb used to indicate obligation, duty, or correctness, typically when criticizing someone's actions.
If an action isn't required, it must not be mentioned. If it's mentioned, it must be required, unless it's labeled optional. That's how I see things working, but only from my observations dealing with gov't agencies. YMMV, but there's a reason we don't allow such "weasel words" in our FAA documentation.
"Should" vs. "Shall" have two very different meanings in legal terms. "Should" is guidance...like the references to the AME Guide. It is optional, but at your own peril (risk tolerance). There are many regulatory "guidance documents" in my field, environmental contamination. We are often at odds with regulatory agencies regarding the validity and usefulness of various guidance documents. There are also many cases of one Federal agency being at odds with the written guidance from another Federal agency. These documents are written by humans, not gods.