You don't waste your taunt at any possible moment. You save it to the moment when dps pulls the aggro off you or when you have to swap targets with second tank.
No, you use it to stack 10-30% on your own agro over and over again before the dps have a chance to pull and the tanking sub forums have shown that that is well indeed possible. DPS should never have agro at any time on a boss mob. by pulling your agro and the other tanks agro out of reach of the dps you are ensuring your job is done, before the boss is even slightly finished. Basically, why the hell would you gamble when you don't have to? the same is said for dps. Don't hold back if the tank can't do his job, your job is to kill the mob before it kills you. Part of that dps is knowing when to agro drop, knowing, when to pop defensive cool downs and knowing where to stand. Tanks need to be strict on their rotation, know when to taunt and if they lose agro, know the best way to get it back and keep it.
What we get from this mentality is a conflict of interest where the dps are actively going all out to burn the boss down and potentially pull the mob away from the tank and the tank holding on for dear life to keep that mob on him. What comes of this is more well established threat and more dps done on the boss. That is ideal.