Full tank are a good idea because they reduce the area of fuel exposed to air. If you have a full tank with the fuel going a little way up the vent, you get a mere square inch or so (pi x vent hose I.D. radius) of fuel exposed to the air. Of course, getting this right is tricky, and you may learn your vent hose is not secured at the tank top properly, but such is the learning
If you leave a tank even 7/8ths full, you have the whole "surface of the pool" exposed to air and moisture transfer from the vent. This is an invitation to getting condensed water
in the fuel, particularly if that tank is nice and cool on the shady side of the boat, and the surrounding air is warm and moist.
You might wish to shut the vent entirely as an option, but I'm not keen on that idea, either.
I have full diesel
tanks on my parking-lot placed steel
boat currently. The diesel is there primarily to keep the tank interiors non-rusted. I will polish it when I get the new engine
installed and the fuel polishing system working, but I don't expect it to be still viable after two years. We'll see.