When my son was in the process for assessing whether or not he was a candidate for his CIs at the Hospital for Sick Children in Toronto, they stressed that teaching sign would not be beneficial to him and would be counterproductive to him learning to speak.
He has his CIs now, and i have never been on board with SickKids opinion on sign language, my son is learning sign now, his teacher has informed me that the hospital is known for having strong opinions against signing when getting implants. I think it is ridiculous, and so we didn't tell them that he would be learning sign while he was still being assessed. Now they can't do anything about it and it is my right as his parent to decide whether or not he should learn it or not. They did indicate in the beginning (subtly) that making the decision to teach him sign could affect their decision.