In order to understand the term of breeding true, you need to understand the genetic behind the strain you are seeking for. Breeding true however doesn't mean you get 100% of the same strain. It is a relative term and a very subjective one too. Some breeders consider 70% of the dropping similar to the parents as breeding true. Some may say 80% and even up to 90%. The other part is the genetic behind a strain. For Blue Grass they can drop Red Grass, Multiple colored Grass (red+blue and in between) and also Blue type (body color variation). But they are Grass strain because similarly a Red Grass will drop Blue Grass because the gene is interconnected. Fancy guppies are selective bred for decades to come to it's current state. It's not as simple as it looks to a beginner and there's a learning curve to it. To maintain a particular strain, you need to diligently select your breeders for each generation to weed out unwanted variation. There's a possibility to downgrading a strain of guppy if there a left to breed ala harem style. So it depends on your final objectives. If you don't mind second grade looking grass patterns then let them be. Hope this helps as I can't explain everything here. You need to do some reading to understand more........good luck on your guppy venture