Personally, I'd be more worried about winterizing the engine correctly than whether the gas tank is full, empty or somewhere in between.
I bet if you polled a large number of boaters, you'd find people are all of the map on this issue and despite their varied winter gas routines, the vast majority have never had a problem. If you're really worried about it, put marine Stabil in the tank before your last outing; although it won't hurt, it's even debatable whether Stabil is necessary for over-winter storage.
I choose to focus on the stuff that probably does make a difference: good winterizing procedures, getting a fresh change of oil in the engine before it sits, and making sure I service my outdrive for the coming season. Gas is the last thing I'd be worried about.