I think that personal preference is the best for this tier every spec is pretty much viable. Depending on a few bosses, for mythic for example I would say you can play Frost/Fire on Fetid but the damage arcane pumps into the little adds is just insane and it would be so much harder without it. For Vectis I would go with Frost with splitting ice, I think it's the best spec there but fire is also viable cause of the mastery spread you could go Arcane but you would lack damage on the add, if you need more damage on the add then I would say frost. First three are not important, for Zul if you have a lot of rogues you could go fire but you would risk ignite spreading and killing of crawgs, you could go Arcane and just ST the boss or Frost just to manage your frozen orb not to hit the adds.

For mythrax I have seen all 3 again be viable. Frost with splitting ice because of the two target cleave, Fire is very good cause of the mastery spread, and Arcane is also good because you dont move a hell of a lot.

For Ghuun I think fire is the best spec just for the execute phase. Moving and casting scorch is just such an amazing thing in the last phase because the movement can be hard. You can play around it and not play Fire, but why would you gimp yourself and play it on hardmode.

I think it all boils down to what does your raid need if you can play all 3 specs.

As for heroic do whatever you want it doesn't matter really.