I know you already have your answer, but I wanted to let you know you're not the only one!
I have a set of UFO rail fence quilts that I started 10 years ago. I just picked up the completed strip sets for one of them, and I'm SO glad I measured before I started cross-cutting for my blocks! It seams (haha) that my scant 1/4" wasn't so scant 10 years ago, and my strip sets (full width of fabric) were pretty uneven. I just cut my blocks to be 6-3/8" (the width of my strip sets) instead of 6-1/2" (that they were supposed to be), and when I put the top together, everything was perfect.
I'm hoping that the strips for the other quilt won't be so far off, since the sets aren't finished yet... Will see!