I'm going to re-echo my previous post in light of the "top tier" post above. ALL GAS COMES FROM THE SAME SMALL HANDFUL OF REFINERIES. Yes, there is some differentiation in the higher grades of gasoline in terms of proprietary additives. But unless you're buying premium gas, gas is gas. The 87 octane you get at ExxonMobil came from the same refinery that the 87 octane gas you buy across the street for three cents less.

As another post points out, because the cheaper station might sell more gas, the gas could conceivably be fresher.