I think the reasoning for never telling someone about their potential immortality is that they might become reckless/careless. Simply knowning beforehand may lead them to become immortal; whereas, had they not known they might have lived out a normal life and never become immortal. So I think it's just a matter of immortal ettiquette. I don't think knowing about the possibility would void it.

And I agree. Sarah would make a lovely Winchester. (But then again, I've also been reading the 'Two Guys, a Girl, and a Chevy Impala' series in which she becomes a Winchester...so I might be biased).