I'm definitely of the opinion rough seas can happen at any time and we've had some nutty weather/seas on the Bahamas/Caribbean sailings. I also think it's a matter of perspective as ten people can be on the same ship and opinions/experiences vary from "we almost sank" to "it was nothing at all". I don't mean to take away from anyone's experience but I've seen it happen and that's ok as people just have a different comfort and threshold. On our Norway/Iceland/Scotland sailing after we had left Iceland and were in the North Sea (as my memory recalls) we hit some pretty rough seas. The waves splashed up against our deck 2 window and it was pretty wild. My DH was LOVING it however walking around the ship there were several people sick, kids having to sit down as they were too ill to walk, people looking green, etc. We were totally fine and not bothered at all but it was a little challenging to walk.
I wouldn't let the itinerary scare anyone away if you had your heart set on it, as again, we've had some pretty choppy seas on our Bahama and Caribbean cruises as well. The ocean is pretty fierce and mother nature is amazing. Ships are made for this and that's what I say when I need to.
Heather