There are exactly two instances I can think of off of the top of my head where it is okay to taunt a mob off of another tank.  Hopefully you guys will help me think of a couple more.

1. Boss strategy. When a boss encounter calls for it, like Vashj.  When the main tank gets rooted (pre-Warbringer anyway) or stunned during Phase 1 or 3, it’s important for the OT to immediately pick her up.
2. To preserve another tank. When you might be out of range of healers and the tank that has a mob is about to die.  A good example of this is the Infernals that drop during Kaz’rogal and Azgalor trash in Hyjal.  Another example was on our Illidan kill last night; one of our Flame tanks went down and I stepped in long enough for the first one to die, then the living Flame tank taunted it back off of me.

This means that if the situation you find yourself in doesn’t fall into one of the top two categories, do not taunt off of another tank. I don’t care if you’re a better tank or better geared or whatever the excuse is, it is a slap in the face of the other tank if you do this.  On a very basic level you are saying to that tank that you do not trust them to do their job.*

We all accidentally taunt, just because you do doesn’t make you a terrible tank but please apologize for it.  A quick, “sorry about that, Bob the Tank” in Vent can help smooth it all over and stop a potential flare-up from occurring.  And believe me when I say that they will know what you’re apologizing for.

There is one situation in which taunting off of another tank’s target is acceptable, though it still isn’t the best idea in the world, and that is AOE tanking.  As far as I’m concerned in these hectic situations as long as the mobs are not on the DPS or the healers it not a problem and you should not take someone taunting off of you personally, you should be happy that you are all doing your job and not allowing the raid to take unnecessary damage.