IIRC, federal law mandates that all unleaded fuel have some amount of detergents and the like intended to keep the fuel system clean. That doesn't stop me from throwing a can of SeaFoam in every 5,000 - 10,000 miles just to be sure.
Suggest to run the octane that your car was intended to burn. For most, this is 87. Running a higher octane fuel can sometimes work against you, leading to unnecessary carbon buildups.
One good thing about the ethanol-blended fuels - shouldn't need to worry about fuel line icing

~ Even though it will decrease mileage by about 3% - ahh, the things we do for clean air.
IMO, the best thing you can do for mileage is to adjust your driving habits. Easy off the line, anticipate stops to minimize brake use, and keep your speeds within reason. A car in good tune also helps - chase after those 'check engine' light flags, do regular maintenance, etc.