On my protection spec I put points into Drums of War so I get to use Pummel and various shouts for 0 rage. Because of this I made a simple macro for combining Demoralizing Shout to Heroic Strike and Cleave since I always forget to keep it up. It's really helpful in allowing me to take 10% less physical damage.
It will trigger a GCD, but as a tank I don't see it as a problem because missing a little bit of DPS doesn't even matter. If you use it when the GCD is on cooldown, it won't re-apply Demo Shout and it won't retrigger your GCD. Taking less damage is more useful to me than doing a tiny bit more DPS. It helps a lot when you're tanking adds like on Maloriak or Nefarian.
/cast Heroic Strike
/cast Demoralizing Shout
If you don't have Drums of War, then I don't know if you want to be consistently using 40 Rage every time you use HS or Cleave.