I think it can be helpful when you’re past 6 cm and I think most studies back that up as well. Sometimes that bag of fluid gets in the way of really pushing the head into the birth canal and it has to be broken (it just can’t apply the same pressure a skull will). I don’t think this matters first time or otherwise. I don’t know anything about the amniotic sac preventing tearing. If it’s not already broken it almost ALWAYS breaks while we’re pushing, en cul deliveries are rare — not sure there are studies on that, and an en cul birth tends to be different than another birth (they don’t tend to push as long in general)… so is that the sac, or the type of birth? Who knows. Gotta take each situation as it presents.