According to wikpedia, the volumetric constant of thermal expansion for gasoline is
For example, if the temperature changes by 20K (20°C; 36°F), the volume changes by a factor of
950*10^-6/K * 20K = 0.0192
The warmer fuel has a volume increased by about 2%, and since energy content depends on the mass, the energy per volume decreases by about 2%.
But this doesn't mean you get more for your money in the morning. Most fuel stations have their tanks under ground, where the temperature is quite constant over the day.
Temperature in the tanks of the gas station can change significantly when they are refilled from a tank truck, but since you usually don't know when the truck comes, nor how much new fuel is dumped into which tank, you can't save money here.
At least in Europe, more and more gas pumps are temperature compensated, i.e. they don't display the actual volume, but the volume the fuel would have at 15°C (59°F). In this case, there is no way to get more for the money.