I wouldn't bother with this, if I were you. Ever since they removed Brutal Swing from Warrior and forced them to use up a role action slot for a stun, I stopped bothering with it. If something needs interrupting, I won't be the one to do it. Let the DPS do it, for all I care. The other role actions are just that more valuable, and even in solo-tank content, it's not worth the hassle that is changing up your role actions/hotbars.

That said, there really isn't anything that you absolutely HAVE to stun, except maybe the final boss of Void Ark, A4S, A7S, and maybe some other rare cases.