The most important factor is how the fabric is folded in half as RkayD says. Not at all familiar with the Fons & Porter book mentioned but one of the ladies at my LQS was cutting strips one day early in my quilting career. And she's excellent. I asked her how she managed that. She showed me her technique for folding to make sure the fold is on the straight. I've adapted a bit but basically start with fabric selvage to selvage. Look for a 'bubble' (easiest done if you're a pre-washer and after ironing). I lay the fabric flat on my cutting table with the fold closest to me and then smooth out the 'bubble'. At that point the selvages will not line up but your fabric will be folded straight. Trim off the wonky end and start measuring from there. Haven't had an issue since.