Dropping Grit for big pulls is a big no no, since you when you have actually stop with all the mobs they actually hit a bit hard for the first few hits so rather have Grit up to help your healer out, and spam Dark Arts + Abyssal Drain with Blood Price up = The most beautiful action sound for me personally in the game. On bosses it's safe to drop Grit after 2 Power Slash combos if you cycle through CD's appropriately since DRK have insane amount of aggro generation and your healers don't mind you dropping Grit (there are some that point blank refuse to heal you unless you have it on, though you safe since your friend is healing).