Anytime I'm on the ship is the right time. Seriously, you can have rough seas anytime of year. But the QM2 is an ocean liner built to handle almost anything and there is little movement. If you are concerned book a cabin midships on the lower decks. You will be so busy having such a great time you will be amazed how quickly the time goes. The ship crossed from NYC to England in early January of this year and it was smooth and apparently a fantastic trip. It could have been the exact opposite. My question would be what are your plans when you arrive? Weather at your destination could be key to what you are planning.
Sun Princess (2 times), Mississippi Queen, Dawn Princess, Coral Princess (2 times), Regal Princess, Grand Princess (2 times), Diamond Princess, Maasdam, QM2 (17 times)