It would help if there was some consistency. I commute to a station called St James’s Park (that’s in London, England, by the way). Some of the station signs say St James’s Park but others say St James’ Park.
For the records, I and the sort of bloke who always adds an apostrophe s for possession where the word ends in s or z or x.
Most people go by William Strunk’s The Elements of Style (later updated by E. B. White and known to those in the writing trade as “Strunk and White.” Strunk’s original text is at Bartleby.com; on this issue, White only added some elaboration.
I question why it’s so “awkward” to have three sibilants in a row, even when separated by vowels, to the point where people feel the need to deform English grammar and make it look Chinese or Navajo. These languages don’t have a genitive or possessive form; they indicate the possessive by simple apposition. Thus in Chinese syntax to express the meaning “the thought of Chairman Mao” they simply say “Chairman Mao thought.” Not “Chairman Mao’s thought.” If I hear someone say “Jesus beatitudes” I feel like I’m hearing someone whose first language is Chinese.
I don’t see any problem with pronouncing three sibilants in a row as long as they have vowels in between to separate them. Unless it makes people feel uncomfortable to hiss like air escaping from a leaky tire. But I’m for always using the possessive -'s regardless of the name’s final consonant.
Oh, but it gets even more fun if you dip into AP Style (which, admittedly, has some pretty assinine rules). This one deals with common noun possessives, though, which is a little different than your example, but does address the triple “s” issue:
Of course, AP’s rules on proper nouns are different too. They advocate adding a simple apostrophe after singular proper names ending in “s.”
Does anyone actually pronounce posessives without the final sibilant? I write s-final possessives with only the apostrophe, but I pronounce them as expected. I’ve never heard anyone say them otherwise.
Lama Pacos, I first noticed this on “The Jezebel Spirit” from My Life in the Bush of Ghosts by Brian Eno and David Byrne. It included an actual recording of a preacher with perhaps a Southern accent performing an exorcism. I heard him saying “Out! In Jesus name.” No audible trace of a possessive. It looks Chinese like that.
“Elaboration” like altering Strunk’s writing to conform to the wholly imaginary rule he added about “that” and “which”. That takes a certain amount of balls, you know? He takes his old mentor’s style guide and then decides that the dude’s writing just isn’t up to snuff and corrects it . . .
Ahh, prescriptivists . . . y’all are a funny bunch . . .
Anyway, I say “Jesus’s”, which leads me to believe that at least in many dialects that form is perfectly grammatical. So I maintain that my writing ought to reflect that. There’s lots of people who have purported to create rules for which singulars ending in sibilants should take “-'s” and which should take a bare apostrophe, but as pointed out above, those rules tend to differ substantially from one another.