Flooring it does help in that the higher temps will help burn off carbon buildup. This is colloquially called the Italian Tune-up. I floor it about once every other week using TT gas.
But generally yes, the best way to use the cleaners is at low load, low RPM.
Depends on your engine of course, b/c my A25A-FKS engine uses port or direct injection in low (interstate cruising) and high load situations, respectively. To get the injectors cleaned, it does need occasional high RPM.