When Is It Appropriate to Ask Someone to Take Your Class?

I have been thinking of when and where I should request someone to cover take my class for me. Is it ever acceptable, such as in situations of emergencies or scheduling dilemmas? I understand that it may sound unethical, but what about the cases when it is only about attendance or taking notes? What is the general attitude of schools or teachers to such a scenario? I would be interested in knowing your opinion or experience on whether this sort of request is acceptable or not.