dealing with curved ribs in a flat rib recipe
Different cuts of ribs have different degrees of curve to them. In my experience, baby back ribs are the most curved, and St. Louis cut spareribs are the flatest (although that is a relative term). This page has a chart showing where the different cuts of ribs come from in the rib cage in case you're interested.

I wouldn't try to flatten the ribs; I can't imagine of any way to do that other than breaking the bones.