Msugi, I've done it both ways. A10N, I've off tanked when the other guy looked at me and said "want to off tank? You'll do more damage." I've also main tanked when the other guy said "hmm, you have more HP and you're pulling threat off me in Deliverance."

I've also been MT and OT in A12N in similar situations. One time the other tank handed me MT reins after a wipe and with my HP pool and use of cooldowns I gave the healers an easier job of it.

It goes both ways but communication and cooperation are key.