You've probably heard that filling your gas tank up in the morning will net you more gasoline because the gasoline will be condensed due to the lower temperature.
To put this rumor to rest, buying gasoline in the morning doesn't have any benefit, other than having a full tank of gas. Gas stations store their gas in large underground tanks that are insulated, meaning that the atmospheric temperature won't affect the temperature of the gasoline.
One good idea regarding gas is to fill up your tank when it's at about one-quarter tank full. This way, you won't have to worry about having enough gas when there is unexpected traffic or emergencies.