When preparing your boat for winterizing, don't top off the gas tank. Leave the tank 1/2 to 3/4 full and add the appropriate amount of fuel stabilizer. (Adding fuel stabilizer is a routine part of our procedure for all boats that we winterize.)
In the spring top off your tank with fresh gas as soon as the boat gets back in the water. During the winter months, gas deteriorates and can damage your engine when your run it hard in the spring. The fresh gas greatly reduced that possibility.
The main reason we all used to top off our gas tanks was to prevent condensation from collecting in the tank with the change of temperatures. If your boat is equipped with a fuel filter/water separator, as all Grady-White boats are, any condensation that might occur over the winter is easily removed.