All of the distilleries have grain programs and feel they are getting the best grain available. At the same time, they'll also admit that the basic specs are the same and many distilleries buy from the same sources. I think what's perhaps confusing here is that no one is using a special grain variety or grain grown in some special way. No one is using grain that can, objectively, be called better than the grain anyone else is using. It's pretty much #2 corn, #1 wheat and #1 rye, and everybody, including the brewers, gets their barley malt from the same maltsters.
Grain is basically a commodity. The most important characteristics are moisture content, integrity (whole, unbroken kernals), and the absence of any defects, such as mold.
So, with that clarification, "select grains" is mostly hype and when words like that are written in marketing copy, it's usually because the writer is ignorant and figures claiming the best ingredients is always a good claim for any comestible. I always laugh when I see it. It tells me I'm not going to learn anything interesting if I continue to read because the writer doesn't know what he or she is talking about.
Maybe the simplest way to say it is that while quality grain is important, all of the American whiskey distillers have mastered that challenge, pretty much. It's not a significant point of difference.
To perhaps go further, "quality" itself is not really a point of difference among American whiskey distillers. I may prefer brand A to brand B, but not because brand B is lower quality. It's just that brand A has achieved a taste I particularly like.
To the extent that relative quality is even a relevant concept, it's reasonable to say full aging is a quality issue, so is over-aging. In the past, some distilleries have had problems with scorched grain, although you never hear of that today. Otherwise, it's mostly just what you like.