If the boss's HP is less than 15% or so, I say "go for it." Everyone else can go AFK - make coffee, do laundry, have a quickie while the tank finishes it off. At 20% or more? Just jump off and reset.

It commonly isn't the tank's fault that the other 3 or 7 party members died. Maybe the tank feels they are being held hostage by people that don't know the mechanics.