It's a good thing to do while the teen is stable and thinking clearly.
Although my difficult child isn't bipolar, he has a enough awareness of his issues that he needs someone (his parents) to be his safety net.
We have done advanced directives for our elderly parents when they were healthy and thinking clearly. Much less drama and emotions. Good common sense works wonders. Doing this for a beloved child seems a no brainer. I don't make decisions without difficult child's input and agreement. If I put my son in the center and he makes a poor choice he knows with a certainty that I will not force him and I will not save him. Like I said, this is a discussion when he is aware and thinking clearly. He is actually afraid of not having our input but my difficult child isn't prone to dangerous behavior. Just stupid.