Well, no, it's not that simple. Different authorities disagree. Derivationally it makes no sense (OE genitive -es would be added on in those cases), and it's practically unheard of in academic writing to omit the "s" after the apostrophe unless the term is a plural (or, apparently, Jesus or Moses, but that's a bit outside of my experience.) The rule's honestly pretty simple: if you don't pronounce it without the final "s", don't write it that way.

(EDIT: And because dragging a music thread off topic is only so hilarious, Edge of Disgrace by Booze Design.)