The temperature for underground storage is the average temperature above ground.
Farmers all over the country kept their garden produce in underground cellers because it wouldn't freeze or spoil because the temperature stayed constant year around.
They went pretty deep with these cellars, though.
Snopes hedges on this question and says the average effect of timing your daily purchases of fuel would be “small.” But how small? I think we would want to err on the side of too much information to the consumer since the sole purpose of the petrol is to deliver energy to the car.
http://www.snopes.com/inboxer/household/gastips.asp
And, what are the air circulation parameters to the cellars and how good is the thermal conductivity to the surrounding soil? They don’t seal gasoline storage airtight because they want the fumes to be able to ventilate. I can personally vouch that the temperature in a basement in the dead of winter in Chicago is 40 degrees, but in hot summer it is about 70, without the contribution of HVAC.