Seems it's nto quite so cut and dry as that, Fraggle... Perhaps the newspapers started using this 20 years ago, because children were taught this 35 years ago.
OK, I should have said, "Check any widely respected style manual in your country." In the USA that would be the MLA or Strunk & White, period. I am a professional editor these days so it's my business to know this stuff.
Forget the newspapers' manuals, no matter how many magazines may use them. Most of them have foresworn the atrocity of "Mr. Gomez' sombrero," but not all. Many executive editors might still overrule their line editors and allow the omission of few instances of S to shorten an article and leave twelve more points of space for a paid advertisement.
As for universities, considering the quality of their latest crop of graduates, who on the average read and write at what my generation regards as the sixth-grade level, I'm not sure I'd trust the standards of any of lesser stature than Stanford and Caltech. When my wife wrote her master's thesis in 1987 on Gabriel García Márquez, she had to fight with her department head to be allowed to write "Márquez's" correctly.
And I don't pretend to speak for other Anglophone countries. I would bet that the Canadians take Strunk and White as a package deal with Big Macs and Desperate Housewives, but I don't really know that for sure, since they adhere to a few relics of British usage such as "zed" and "theatre." The USA and the UK make it a point of honor to disagree over linguistic matters whenever possible, and I don't see enough writing from the Antipodes and the semi-anglophone countries like South Africa to hazard a guess as to their practices.
I humbly submit to Wikipedia's wisdom on a few universal exceptions that I simply overlooked, such as "for goodness' sake" and "Jesus'." But those are indeed exceptions and rare ones. The general rules as I stated in my original post should be followed unless a respected authority in your own country tells you otherwise.
(Grammar rules DO change sometimes in an evolving language, as you well know better than most).
Yes, grammar rules change because they reflect the spoken language, which until a century ago was totally ephemeral. But the rules of written languages change much more slowly for the obvious reason that the writings of our ancestors are still widely read and exert a conservative influence. Inconsistent spelling and punctuation make for difficult reading.
Look at the saga of the serial comma. (The second one in "Life, liberty, and the pursuit of happiness.") I believe it started falling out of favor in the 1940s or maybe even the 1930s, perhaps as a miniscule effort to conserve newsprint just like "Mr. Jones' hat." I think it began infiltrating the style manuals in the 1960s, about the same time New Math began destroying the numeracy of a generation of children. I don't know what sparked the counterrevolution but the example I always see is the book dedication: "To my parents, Ayn Rand and God."
Today's documents full of technical jargon, such as the ones I edit, would be illegible without the serial comma. So it is now optional, but only if the writer is positive its omission will cause no misunderstanding. I predict that in another generation it won't be optional any more and its absence will be a historical curiosity from the same era as the hula hoop and missile-shaped auto bumpers.