I've heard that the midship section is more stable, too. But honestly, if there are rough seas, the whole ship will rock. You certainly don't want to spend your cruise locked in your cabin worried about being ill. Be proactive. If you're worried about the motion, take something ,or try the sea bands.
If the motion of the cruise ship bothers you, think carefully about the shore excursions you choose. Many of the smaller boats cause seasoned cruisers to feel queasy, it's a much different sensation.