Like those commercials for JGWentworth, its your money, use it when you want to. Page 223 of my owners manual says 10% ethanol use is permitted. Page 224 of same owners manual under item "Materials added to fuel" states "Use of additional detergents or other additives is not needed under normal conditions and would result in unnecessary cost". So, its your money, use it when you want to! I do use a product to remove moisture in the fuel tank about once a year. You never can control the water content of the fuel you buy!

Only additive I use regularly (every oil change) is engine oil treatment. I use it because it coats moving parts and sticks to it after long periods of inactivity. It keeps those parts coated for when the car is first started (where most engine wear occures in normal use) until circulated oil replenishes the entire system. Sure wished cars came with an electric oil pumping system that would kick on for a few seconds before engine start (maybe wired into the 'opening' of a drivers door, or door unlock!).