According to https://en.wikipedia.org/wiki/Top_Tier_Detergent_Gasoline you're getting at least the EPA minimum amount of detergent additives even in low-octane fuel, and many brands participate in the Top Tier standard that requires even more additives, again with the requirements applying even to low-octane fuel. So I wouldn't bother paying extra for premium fuel for the sake of additives when the higher octane rating isn't needed. Which gas station you go to will probably affect that more than which grade of fuel you select at the pump.
There is generally one refinery that all stations in a given town buy from. Shipping fuel from a different refinery would be enough more expensive that you wouldn't be competitive.
Sometimes stations have their own additive package.
