Yes, punctuation rules are fluid, and yes, they do vary from one grammar text to another. Thus, on the rare occasion that I beta another's fic, if I see "James' balls," I withhold the red ink and say, "I guess she was taught differently."

However, it does make my skin crawl just a teensy bit. And the students I teach are taught the rule: "Add 's unless the word is plural and ends in -s, and then just add an apostrophe."