Recently I conducted an experiment on the topic of when to add dark grains to the mash. Some recommend holding out the dark roasted grains until the very end of the mash. A number of reasons are given. One claim is that by holding out the dark grains one does not need to add any bicarbonate salts to the mash to counteract the acidity of the dark grains. This is a reasonable claim, especially if your water already contains a significant amount of calcium or sodium. Since the most common bicarbonate salts are calcium and sodium bicarbonate, adding either salt will increase the levels of sodium and/or calcium. But my water is already low in calcium, and it does not have much sodium either. I am usually looking to increase my calcium levels, so adding calcium bicarbonate to the mash does not bother me one bit.
A second claim I have heard is that allowing these dark roasted grains to mash for an hour is akin to letting a pot of coffee sit on the burner. After an hour the nice roast flavor will become burnt and harsh. If that were true, then waiting to add the dark grains would certainly be a no-brainer.
I just finished the last two bottles from the experiment, and I can say I found no taste difference. As always, your experience may differ, but I plan to continue to mash dark grains as I always have.