You don't need to spend money on additives unless there's something specific that it might help with, or if it's actually needed to run the car, like lead substitutes for engines designed to run on leaded petrol. A higher octane rating won't do diddly squat unless your engine needs it because it is pinging, and even then E10 will be cheaper and probably more effective than running a dubious octane booster with every tank. Only additive I ever used was an injector cleaner as a one-off. Have a read of the
fuel bible, it's got a pretty good discussion of all this stuff.