During the 30s a convex rounded end was in style (think 'Dymaxion'), but more lately a sharp crease for fixing the point of separation is in. See the crease added to the back bumper of the latest Prius.The panel inside that separation line can be convex, flat, concave or a recessed box. Concave or convex would be stronger for a given weight than flat, or a flat box.
Here's an example (I like) of a concave or boxed rear end.
http://jalopnik.com/the-renovo-coupe-is-an-electric-shelby-with-1-000-lb-ft-1621969757