Adding fuel additives every tank full beyond what is already in the gas will kill your O2 sensors and cats,both are not cheap.
x2. I used a bottle of injector cleaner at 50,000 miles, then another one at 70,000. However, I have to admit I noticed no performance gains or general smoother running. I'm reluctant to use fuel additives often because as tj says, you're going to toast your 02 sensors if you use them regularly.