Re: fuel additives
IMHO, personally it is an annual thing for a couple reasons:
Even with top tier fuels, there are still deposits accumulated on injectors. Some of these are actually detergents left behind when the fuel flashes off at the nozzle itself. I have a friend with an injector flow meter. I have seen the results of before/ after on my race bike injectors. Granted, the engine spends its life at 15,000 rpm, but it uses top tier pump fuel.
Valve overlap. There are times in the combustion cycle when both sets of valves are open to exhaust gasses. The carbon build up can settle on both intake and exhaust surfaces. It accumulates on piston crowns too. Sure, there are detergents in fuels to help prevent this. They do a great job when you are doing highway cruising. However, how many of us really spend more time cruising over stop n go traffic? Acceleration and deceleration are times where incomplete combustion happens most often.
FI cleaners, especially the ones formulated for ethanol fuels, do a good job of cleaning up what gasoline detergent leaves behind. They generally work by burning hotter and "cooking off" the deposits. They also help rid the fuel system of corrosion deposits left by moisture trapped within out corn based alcoholic fuel.
Like I said, it is an annual thing for me. It doesn't hurt, it's cheap and since I tow a good bit, there is a better chance my combustion is not as complete as it could be.