Wow! Interesting question.

Well, as a general rule, apostrophe -s is added to all nouns (including proper nouns) when referred to in possessive form. But, of course, this is very much debatable in special cases such as this simply because sight of multiple number of "S" in the word is unpleasing to the eyes. For me though, I drop the S if the last letter of the proper noun is already an S or a Z. I remember when I was a kid,my ELA (English, Language, Arts) teacher taught us that an apostrophe's purpose is to replace any missing letters. Hence, it's safe to use just the apostrophe in these cases since it stands as a replacement for the letter S.

But then, who am I to say. Sariling opinyon ko lang naman ang lahat ng ito. Hehehe. Hindi ako nagmamahaling kaya feel free to correct me if I'm wrong about this.