I rend when either a) there is some sort of air phase (Sindragosa) or b) a tank swap fight, where I'll throw it before I taunt to help keep me over the threshhold.
As has been said many times before, using that one GCD before an air phase to cast devastate/SS/revenge instead will always be more threat. Your second point is the only situation I can think of where rend is more threat because you used the GCD before taunting to do it.
One thing I should have mentioned though. Rend probably isn't going to hurt you too much, just make sure you don't have that stupid add-on that proclaims you've managed to put rend on a raid/world boss.