Prism99 is correct - grain line is essential in clothing but not as much here in quilting. I always line up the selvages, check to see if the fold lies flat - if not then I do as Petthefabric suggested. Then I square my first cut to the folded edge.
My only deviation to this is when I want to follow a pattern in the fabric (such as stripes etc) then I fussy cut regardless of the grain. Hope this helps!!