Its really hard to state " various strains of discus that breed true." Your older classic lines Can breed true, fish like cobalts, Red Turqs, Blue Diamonds, many Pigeon strains. They will breed true if they haven't been outcrossed in the previous generation (s) , so they have the potential, but alot of times a discus looks like one strain and carries the genes of others.. Wilds will breed true obviously in most cases though. Your best bet to get fish that will breed true is to work with a breeder where you can see the stock they are working with.
al