Except none of this matters unless you're running into a problem with enrage timers or failing because the group is running out of resources (both of which would be a DPS-role failure, by the way.. not a tank-DPS issue, as battles aren't tuned towards tanks putting out extraordinary amounts of damage). So long as things are progressing smoothly it really doesn't matter if the boss dies now or 30-60 seconds later outside of satisfying some groups lack of patience.
And should things hit the fan for some reason, usually it's better to have a tank with extra health on your side rather than a tank putting out a little extra damage.