Have never had a problem with cold weather evaporation. Always the same level in spring as I left it. Also no rust on the inner tank walls from condensation with a full tank. On a frosty morning you can look at a gas tank and see the fuel level by the frost line above the fuel level. This is exactly what it looks like inside the tank and this in turn turns to water. I will fill the tank.
Even my fluid filled tractor tires will have a coating of frost above the fluid level on cold damp frosty mornings.
But do what ever works for you.