Fuel injector cleaner works better if you pour it into a nearly empty tank because there is less gas to dilute the cleaner. It will do a quicker job of cleaning the injectors with a higher concentration of cleaner. If you are using gas treatment there is a lot of alcohol in it to remove any water in the tank. If you don't dilute the alcohol it can degrade compontents in your fuel system.

Of course if you are running with SU's the odds are that there was going to be an overhaul in the near future anyway.