I don't have a single heater going and i raise a bunch of fish like apistos, rams, discus, etc... it stays about 68 degrees in the fish room, sometimes as low as 65-66. Honestly i think it is a waste of electric.
I'd like to ask how often your discus and dwarf cichlids are breeding. Just because they haven't died yet doesn't mean that keeping them at grossly inappropriate temperatures is a good practice or something that should be recommended to others. IMO buying fish and then refusing to do what is needed to keep them properly is more wasteful than what you're proposing.