In dungeons I drop Grit against bosses because they spend a lot of time using pathed attacks & not actually hitting me, pop Shadowskin/wall to eat their bigger attacks. I usually leave it on during trials and raids so the healers don't waste as much MP on me & have more time to DPS the boss. Exceptions for cases like A1S first phase or Goblin packs in A2S where my short term DPS gains outweigh any advantage from staying in Grit.