This is an interesting question. I'm in the unique position (I think) to know the family of a former member here who ctb. She told her family repeatedly for the prior year that she was rational and committed to ctb at a near future date. Everyone knew but they couldn't really do much except watch over her, which they did, well and lovingly (and she felt their love and loved them, but her life was in ruins, outside her control).
Interestingly, she agonized a lot about method and timing, and finally realized there was no way she could do it without a member of her family finding her. Since the progression of her illness she'd always been with family so going away would have raised immediate red flags.
I know from reading all her writings, she felt a bit backed into a corner by this. But did the best she could as far as trying to tell and prepare everyone. Every family member I talked to said she'd been talking openly about it for some time, which did upset them, naturally.
I guess for me, I've thought through whether I'd like to warn people or not, but then I look at this person I knew. I am quite close to several members of her family, and you know what? I'm not sure all that preparing and thinking of them did any good. They were/are still shocked, angry, felt betrayed, etc. I've heard from 2 family members that it's horrible that she picked the person she did to find her. And this was several years ago.
So my conclusion is that when it comes to suicide you're damned if you do and you're damned if you don't.