Car Talk had a caller recently that posed this in the form of a question. The response back was yes, buying gas in the morning provides you a little extra gas. No, the little extra gas is not worth worrying about. The upside was less gas fumes emitted to the air by filling in the morning. The downside was potential overfilling and gasoline spilling out of your tank when it expands later in the day.

Gas boycotts do more to hurt the vendor than the suppliers or corporations...the vendor makes most of its money by selling the goods in the store...no gas sales historically means no candy/soda/beer sales.