For trash, either tank or myself or anybody I think will pull aggro. Trash doesn't do much damage so much of my single-target is overheal so if someone pulls aggro it could mitigate some damage as they're being hit. Idk just personal preference.
For most bosses, I just slap it on the tank
One exception is HM/NiM Dread Guards in the third phase where I place it onto a dps or healer who has withering terror since that will help mitigate some of that damage while it prevents another health proc on a force leeched tank, potentially killing it.