I am a veterinarian in a practice that has had 2 technicians in training and numerous vet student interns in my 15 years in practice.  All have been well supervised and allowed to perform skills based on their training in school and on their competence at particular techniques.  As a Veterinarian, I will ultimately be responsible for the well being of the patient and the outcome of the case, so you better believe that I will not allow a tech, tech student, or vet student to perform something without appropriate supervision.  Requiring written consent is unneccesary and cumbersome. It would dramatically hinder my ability to provide supervised hands on training to develop competent and confident technicians and veterinarians.