[jrockway]

Many style guides recommend the "Bobs's" form over "Bobs'".

Those style guides also mention many more special cases, like "the students' questions" (but "the dutchess's hat", because students is plural and dutchess is singular) and even break that down to the case where the next word starts with s, like "the dutchess' style".

Basically, it is not by any means clear cut.