Yeah, usually when my mind starts to wander, it's reflected in slouching. I don't know which (if either) comes first, though.

BTW, if you're getting hung up on having a perfect cushion, let it go. You can sit zazen on the corner of a couch cushion. Seiza (http://en.wikipedia.org/wiki/Seiza) works, too, but I wouldn't unless you're already used to it.