personally i just use it to adjust to the situation, though im not really a raid-worthy drk. start in grit to get my aggro up and test the waters with damage, if no basic hits are dropping my health too bad, or im not getting hit, i'll gladly drop it for blood weapon, preferably when i have some oGCDs coming off cooldown i can use without grit for the extra damage on. if i'm getting hit fairly regularly but not for much (auto attacks, not busters), i can drop it, go blood weapon, bloodbath and dark dance to get the speed attack speed, the mp, the self healing and a few more bloodbath self heals with stun/ reprisal procs. Though keep checking your aggro in case that sneaky overgeared blm starts catching up, or i make sure i can grit to prep for a buster with a souleater after to help regain that little hp afterwards.

I guess its up to you to decide how you wanna use it. some fights its barely useful, for example in the last phase of sephirot i use grit when adds are up and for maybe 1/3 of the tank soak towers, but in a less scripted or more direct damage fight, it can be safer to err on the side of caution until you know.

long story short, dungeon bosses you can use to experiment with stance dancing as they dont do a great deal of damage, and learn what you can get away with. even if you just like turning grit off to use blood weapon and back on when its over