margin: 0 auto is the short hand for setting all of them, you're right.

The equivs

margin: 0; - top/right/bottom/left
margin: 0 0 0 0;

margin: 0 auto; - top/bottom right/left
margin: 0 auto 0 auto;

margin: 0 auto 2px; top right/left bottom
margin: 0 auto 2px auto;