I am sitting at baggage claim waiting for my bag and noticed that almost all "normal" suitcases (not duffel bags, or overly large hard shells with 4 wheels) are almost always arriving on the baggage carousel with their back side facing up. Why is that? That's normally the soft side of the suitcase so if someone had something fragile packed on top, it would probably break.
In places where I need to put my bag on a conveyor belt myself, there are always signs saying to do so "wheels up". I think that's so the bags don't roll on the belt and the wheels are protected from bumps and drops. Then they just come out on the conveyor like that. Generally if there are only two wheels, they are at the back.
Never pack assuming any particular side will be the top or the bottom. Put breakable stuff in the very middle of the bag, surrounded by padding such as clothes.