I have never liked assimilating the possessive “s,” as some copy styles advice you to do when writing about the things belonging to Jesus or Moses (Jesus’ pinball machine and Moses’ bad attitude as opposed to Jesus’s pinball machine and Moses’s bad attitude). Some even suggest you do so when putting any regular, non-Biblical name ending in “s” in the possessive — Thomas’ menstrual cycle instead of Thomas’s menstrual cycle. It doesn’t make sense to me, because you still pronounce the second “s” even if it gets assimilated. Maybe some people don’t, but I do. And, if you’re writing, I feel like you can take the time to make one more “s” after that little nothing of an apostrophe.
Yesterday, I realized another objection to the assimilated “s.” The name Lenny Kravitz does not end with an “s” as it’s written, but it does as it’s pronounced. If he were Lenny Kravits, we’d pronounce his name exactly the same. By the logic that suggests we write Jesus’ pinball machine, shouldn’t we also write Lenny Kravitz’ unusually located eleventh toe? Of course not. That would be dumb — but really, no less dumb than eliminating the “s” in any other case.
Really, the only time it makes sense not to write out that possessive “s” is when it’s a plural possessive such as in We happened upon the foxes’ brunch. We wouldn’t pronounce it any different if that plural noun weren’t in the possessive case — We happened upon the foxes, brunch.