A quick search and I found, in reality, thermal expansion is more like 1% from summer to winter. Here's why, the gas is stored underground. And winter or summer, the gas is always dispensed at between 60 and 65 degrees. Close to 60 winter, close to 65 summer. The pumps are calibrated to be accurate at 60 degrees, so you do get 1% less gasoline for your money in summer.

It does not matter what time of day you fill up, since it is the temperature in the underground tank, not the outside air that matters.

You should not "top off" in summer, because the gas will expand in your tank... after you already paid for it.... and could overflow if you don't use some right away.

Still, that 1% is extra profit for the gas station in summer.