ReplicatorG has this feature, and its super handy. As far as i can tell, its just "find the biggest flattest section nearest to level, then rotate to be parallel to the plate". I have a lot of models that are skewed to funny angles (like 53.45 degress), and it would take forever to lay it flat by trial and error. Right now im just importing it into repG, using lay flat, then importing into s3d.