Is Tyrantrum actually confirmed for having Rock Head? I can't find it anywhere and the community seems uncertain. If it were my choice, though, I would run Head Smash with Rock Head on a DD Tyrantrum. Moves like Fire Fang and Crunch are mostly there for coverage, so if you ever have to resort to those that's because your STAB major power hitter Head Smash isn't an option. Think of it along the line's of DDNite, who uses Outrage on anything that gets hit for neutral damage and runs Fire Fang for coverage. If you're going to be using your bite moves for coverage, then there's no need for Strong Jaw bonuses, although I see a possible counter in Ferrothorn in this case with the proper investments. The difference between running Strong Jaw for 2-3 moves and running Rock Head for one move is that the one move should be your major choice most of the time because none of the other moves are going to give you that massive damage output that Head Smash would, and if you're resorting to bite moves you're probably getting a supereffective multiplier anyway.