Also found a few things on the temps of underground tanks.
"While density may change with temperature, underground storage tanks sit 15 to 20 feet below the surface so the fuel stays around 55 degrees Fahrenheit. Bruce Bragg, national account sales manager for fuel-dispensing equipment manufacturer Catlow and a 30-year engineer for a major oil company says one of the only times that you'll find a warmer, less-dense gas is if the fuel doesn't have time to cool off after being pumped into the underground tanks. Fuel temperature stabilizes quickly, so the chances of this making any difference are slim."
http://soilphysics.okstate.edu/softw...e/document.pdf
In the above you will find a chart that shows ground temp changes over the course of several years. at 1.6m the change from day to day is less than 1° C. At .05m below the surface the largest change observed is about 4° C.
Further supporting the idea of not bothering with what time you get gas.