Yeah, I agree with your thoughts on the 95% thing. Its like with pain management. I am completely non responsive to morphine. Most people respond well, it just feels cold in my arm and that's it. When I flayed my wrist they gave me tonnes of morphine, the maximum dose I could have, and I had almost no effect at all. I got more from the paracetamol they gave me after that which was good because they had to remove my temporary dressing from a very large open wound and any relief was helpful. Now I just ask for aspirin and paracetamol, though after a wonderfully fun heart infection I can't use aspirin for pain relief without considerable bleed risk. Oh well, paracetamol it is.
But yes, if I go in for emergency care and tell them "no morphine, paracetamol only" they will probably not take it seriously without a doctor supporting it. Good thing I have a fairly high pain threshold.