Editing The Iron Conqueror, I’ve danced on this question a few times. I own several grammars, and none of them have agreed on this. One of them even makes the amusing contradiction “As a general rule, use ‘s.”
Unless, and then the exceptions come. One of which was “If pronunciation becomes awkward.” (Arguably always true.) Another was, “If including the ‘s would create another syllable.” *facepalm* You either hiss it and sound like a snake. Or it’s another syllable!
So that source just threw out its own ‘rule’ in practice. Even the Grand Dominatrix of Grammar: Kate Turabian imposes multiple variations. And that’s all well and good if you’ve had the misfortune of writing a Thesis according to her slide-rule calculated form, as I have. But most of us, as AUTHORS are not interested in such self-flagellation for non-academic purposes.
So at the end of the day, what do I say on this? Meh. I use James’. But whichever way you do it, be consistent. It’s the only sane approach.