Worst case is a partially filled fuel tank. A completely full tank will tend to collect less water than, say, a 1/2 empty tank, because there is less air space above the fuel for the condensation process to take place. Condensation (sweating) in the top area of the fuel tank will reach a point to where the water droplets will fall or run down the sides into the fuel and become trapped in the bottom, under the fuel, where it cannot evaporate.
So, IMO, there are two best options and of course both also involve using storage stabilizer to help prevent fuel quality decay, but I prefer if possible to remove as much as possible of the fuel till there are only minimum amounts remaining and the condensed water isn't trapped, thus can evaporate off the bottom of the tank.
One way to remove a majority of the fuel is to siphon it or pump it using an electric automotive fuel pump, THROUGH THE FUEL PICKUP, which draws from the bottom of the tank, into several spare gas canisters. I do this for my customers who prefer an empty tank during winter storage, but it does require some equipment such as the appropriate fuel line connectors and associated hardware in addition to an electric fuel pump but I can usually remove most all the fuel by positioning the boat such that the fuel runs into the fuel pickup area.
The other option is to place the stabilizer in the fuel tank fill tank and flush it down into the existing fuel with good fresh fuel, or mix first and fill the fuel tank completely before performing the fogging process. It's key though, to fill the tank to the point where the airspace above the fuel is minimized in order to avoid condensation.
Some people don't like having the fuel tank full due to the fear of fire, mostly in the case their boat is stored indoors during the off season.
