ANytime I vacation in the Caribbean, I always go as early in the summer as possible. The weather in the tropics (on average) does not kick up on a consistent basis until late July to mid september.
I've been chased by hurricanes when I was in The Navy. No Fun. Sure as heck do not want that to happen when I'm spending some big $$$ on a nice vacation.
If I could take my kids out of school for a week, I'd definitely go in the fall (Preferably November, because the tropics are winding down, and the rates are really good)