I played DRK up to 60 before SB. When I tanked I stayed in Grit the entire time. Was I playing wrong? I see people mentioning dropping Grit to do other attacks, but I don't know if that's the proper way to play or if that's the people stacking strength.
So the fundamental tank stance thing is that the defense/enmity boost is only any good if you're getting anything out of it. There are generally a lot of situations where you'll have a decent threat lead and not be getting hit that hard on average, and can safely leave Grit for a while.
The end-endgame meta in HW was largely treating Grit like another defensive CD--you pop it when you need the extra defense, drop it when you don't (which is most of the time in raid content.) It takes time to get accustomed to, but the general idea is if you're going to tank for a long time you want to get familiar enough with all the pieces of the kit and all the tradeoffs involved to know when you do or do not need them.
It's not a thing to focus on while leveling, necessarily, but given that once you've established a threat lead and figured out where to cooldown to not die there's not really a lot of tanking stuff left, figuring out the minimal tank stance/max deeps stuff usually ends up being the only avenue for long-term growth.