Another thought... what if FF is the slayer? Same question goes for anyone we pick. Obviously, that person then has no choice but to either use the stake on a hunch; or wait to use it. Both of which I think are bad ideas.
I hate to sounds like a negative nelly, but from work I've been trained to look at worst-case scenarios vs. maximum utility.
I see waht you are saying and understand your caution. Short of the slayer coming out (which I am not suggesting they do!) we would just have to go on faith and our best instincts; some better then others!