It depends on what you want....We've done several fall cruises, one summer, several Jan/Feb. The only downside to the fall is that it is hurricane season. That adds a little risk that the ship might not go where you thought you were going to go, but as the captain advised us, "Only a fool would steer a ship into a hurricane." I can tell you that we had a wonderful time on our 2 "re-routed" cruises. Actually, Nov. is the end of hurricane season, so you have a smaller chance of a "creative" trip. Some fall cruises are marked down as they don't always fill.
I found summer to be too hot to enjoy, so we don't do that any more.
Jan and Feb are also "value" season, but it can get cold (one our of 5 cruises was too cold to swim...so 4 of our 5 were good).
So...there is really no bad time. School vacations tend to be a crowded time with lots of school aged kids, off season has more pre-schoolers.....