Unless your dog has a specific sensitivity or allergy to a specific grain, grain-free is just advertising hype. Can your dog do well on a grain-free diet - sure. Can your dog do well on dog chow - sure.
I'm not a fan of Blue Buffalo - any of their foods. Not because they are necessarily bad foods, because their advertising is misleading or just false in many cases. They keep trying to tell the public that byproducts and some other ingredients are "poor" while actually they can provide essential nutrients that are not available in muscle meat. It is true that byproducts could contain useless ingredients (beaks, feathers, hooves, hair), but better grades of food use mon-human grade ingredients that do provide important essential nutrition. That does NOT make Blue Buffalo a better food.
You can push too much protein in growing puppies, especially large/giant breeds. I do believe that most brands formulated for large/giant breed puppies provide the appropriate nutrition when fed appropriately.
I have no problem with Purina Pro Plan large breed puppy food.
Sometimes breeders get an idea from their experiences or from limited exchange with other breeders. The ideas or opinions are not necessarily wrong or bad, but are often based on a limited set of facts. To make good judgements it's best to evaluate all the evidence available, rank it according to reliability, then make your decision. Evidence based problem solving.
Doug & Sandy
Kaylee
Winnie 6 1/2 year old golden
2008 Southwind 2009 Honda CRV