My editor uses the Chicago Manual Of Style and has told me that even with names ending 's' to show possession, it is still necessary to use 's. I really hate that. Is it acceptable to pick and choose what you prefer from different styles in the same work, or must I follow all the rules within CMOS if the rest of my manuscript is that way? I cannot find an answer in my internet searches.
Strunk and White taught the same rule in The Elements of Style, so it has considerable history. I personally favor it as well.
That said, it depends on the type and audience of your work. If it is for inclusion in an academic journal, follow the journal's preferred style strictly.
If you are writing non-fiction for a particular audience, especially in a technical subject, follow the usual conventions of that field, whatever they are.
If you are writing fiction for a traditional publisher, you would do well to follow whatever style book the publisher advises, except where you have a good reason and can articulate it.
If you are doing fiction on a self-publication basis, you have no gatekeeper to satisfy. You need to avoid losing your audience, but this kind of small point will probably not affect many readers. However if you are lucky enough to have a quality editor for such work, it might be wise to take heed of that editor's advice. But converting 'Charles's" into "Charles'"(which the traditional rule says should only be done for plurals) is not likely to be as annoying to an editor as the "greengrocers apostrophe", that is using an 's form for a simple plural, as in "I read five book's this week."
English does not have an national Academy to make iron-clad usage rules, and even in French such rules cannot be enforced.