I'd agree with you Melanie, but I've also seen major errors in a family Bible due to poor recordkeeping as well. So I suppose I tend to rely more heavily on official documents, even though the truth is probably family Bibles are probably equally reliable.
Jillaine - that's an interesting article, and after walking through it's example it makes sense, but it might be too deep for the casual genealogist to use. I think what I see Robin trying to aim for is a quick cheat list of the "usually" reliable sources versus those that are typically "mostly" reliable and on to those that might be questionable or need validation. I guess we've all heard the words of direct, indirect, original, derivative, and so-on, and for scholars this might be the best approach. But I'd have to see a practical way to explain this to a non-scholar that would be easy to use and simple to apply and accomplish the goal of helping genealogists understand when a work is typically accurate and when it's been transcribed multiple times or that leaps of faith have been made.