In raids, I regen the tank as soon as he runs toward the boss. In dungeons, I usually don't know the tank, and I'm a lot more careful with my Regens. I rather throw an instant at him than risk tanking myself; many tanks just aren't good enough, and even if they are good, I feel pre-pull Regen isn't worth the chaos it potentially causes. As soon as the tank finished the pull, sure, Regen is up asap, followed by Holy, Aero II, and all. But before that, no. Too much chaos, too little knowing how the tank will react. Not worth it, I can keep him alive during the pull well enough with other means.