It's a stylistic choice, so opinions vary, but the simplest guide I've found is to simply check the intended pronunciation. If you would pronounce the 's as it

It's a stylistic choice, so opinions vary, but the simplest guide I've found is to simply check the intended pronunciation. If you would pronounce the 's as its own syllable, include it (e.g., "Chris's"); it not, don't (e.g., "Mephistopheles'").