Pretty much all of the above.

It is definitely one of those "play the role to the best of the ability" scenarios. Having it helps - even if you pre-cast and you do not manage to have it up for a major hit, but it is up for the smaller auto hits after, it can be a life saver, or just to make the battle that much easier. I do not use it during dungeon pulls as my mana goes to flash in te majority, but I do on most every boss just because I can and again, every little helps.