As a redundancy check to complement a Boss Mod, I also have the following.
I set up a focus frame that shows debuff Icons. When we approach a boss fight that requires tank swaps, I set my focus frame to the tank I follow in the taunt rotation. I watch for the icon of the debuff they have and the stack count to determine when I should take it. I then watch my personal Debuff bar to see my stacks, once they reach the required amount I hit ESC and sit there for a moment and let the next tank get aggro, before resuming attacks.
Basically your answer lies in a few good addons.
There is something so appealing about backhanding someone across the face with a shield.