Week 1 grades are the best grades: not the most accurate or enlightening, but easily the most fun. There's no baggage, except what carried over from last year (which is often a lot, but never mind). Half the NFL is undefeated, the other half is in crisis mode, and even the weakest team can hang its hat on a 42-14 victory and dream of a better tomorrow.
These are the weekly report cards, not the power rankings. Every week is a clean slate: Play well and you get a good grade, though teams with the same grade are sorted by reputation/perception.