This questions comes up a lot, and there's no clear cut answer.
As mentioned above, Carrol Smith's, "Nuts, Bolts, Fastners, and Plumbing Handbook" is an excellent read. You'll learn more about fastners than you thought there was to learn. He wrote in easy to understand terms, so even I could learn from it.
http://www.amazon.com/Fasteners-Plum...0&sr=1-1-spell
In his book, he states that grade 8 is for clamping only. It should never be placed in shear - even double shear - because it can fracture. The 8's should only be used where there's a dowel to prevent lateral movement.
He goes as far as to say, "There is no place on a race car for a Grade 8 fastner." Not sure I agree with that statement, but there it is.
As far as the seat belt goes, I think if you manage to break a grade 5 or 8, it won't matter. The belts would have cut you in half already.