It really depends on your gear. I often use innocence towards the beginning of a new level cap as your gear will suck. As you progress your gear is often enough cap phys MIT anyway.

However, it's worth noting that the stat caps increase at higher tiers of instances, which is not reflected on your character stats. I believe around 1500000 is the MIT cap at tier 2 or 3 right now.

Anyway, as a general rule I find myself focusing on mits if my gear is.lacking and then morale/vitality as my gear and essences cover the Mits.

As Gor already pointed out your actual force taunts aren't resisted but your shield skills can be for sure. It's a luxury but some finesse is not a bad thing to have as a guard.