Thats one way to limit harsh astringency that can be added by the darker malts, but is not always necessary as you have seen in previous batches. If you are treating your water with water software, when you hit the correct mash pH, you will also avoid those harsh flavors. There will be a lot of responses here and most will tell you to experiment. I made great dark beers for a year before I started treating my water, and never separated the dark grains. Its really a combination of things that lead to the astringency, kind of a perfect storm of things going wrong. Separating the dark grains will eliminate one aspect of that perfect storm, making it less likely