The EPA does mandate a basic additives package for every gallon of road fuel that goes into the pipelines. The more name brand gas companies do add some proprietary additives to the gas once they draw it from the pipeline though. IMO, this is manly marketing.

I fuel up at where ever the gas is the cheapest. My 17 year old SUV with 280K averaged 16+ MPG from new up to the day it died. Zero fuel problems.