1. There is no itinerary anywhere in the world where a ship hasn't experienced rough seas at one time or another. Whether yours will or not is completely unpredictable. When we cruised to Hawaii we had smooth sailing all the way, but we've occasionally experienced rough seas elsewhere.
2. What people consider "rough" varies dramatically from person to person. What you may consider rough water may be just gentle motion to me. If you had been aboard the same cruise as the people who tells the "horror stories," you might well wonder what they are fussing about. Plus you have to remember that some people enjoy embellishing stories for dramatic purposes.
3. But at the end of the day, if you choose to sail upon the seas, you are subject to the whims of the seas. Trying to give odds on the smoothness of a given route is not a terribly productive way to pass time.