I solved the straight strips by cutting a 1/2 in slit in the edge of the fabric and tearing across the fabric, it will always tear on the straight of grain. If the fabric tears off before going to the other side, start on the other side with your slit and tearing. We were taught this in Home Ec. back in 1963, tells my age doesn't it. I even tear strips for quilts, allowing an extra 1/4 inch for the ravel edges then trim off after the first
sewing. I have done this many many times and turn out perfect straight of grain strips, squares etc., just always check your cutting line after a few cuts to see if your are staying straight or have slipped if cutting.
Hope this helps.
Gangles