As they others have said, gas is all six and half a dozen of each other no matter where you go. Buy the cheapest you can find and stick to the recommended octane grade. I'd probably stay away from shady looking stations but all name brand ones offer an identical product.
There have been rare instances of mistakes though. There was a story last week out of Calgary. A supply truck had accidentally pumped diesel into a station's gasoline reservoir. The station owners at least stepped up said they'd pay for any repairs.