I usually rip borders lengthwise, but before I got up the nerve to do that, I cut single layers, also lengthwise, but I slid the cutting mat under the fabric instead of sliding the fabric on top of the cutting mat. It was much more accurate and I have a seven foot long table so it was easy.

folded, especially if you are cutting from length of fabric so it is really long. it does help to cut one strip, then turn the fabric over so the next cut is started on the opposite end than the cut before...this helps keep things straight. if you cut strip after strip in one direction they can become wonky. the same goes for sewing long strips together (like bargello strips) if you don't change direction between each one they start to curve (quickly!)

I tear my borders have for 25+ years. Tried cutting hated the results, not as straight. I tear lenght wise. You will hear many say it distorts, not if you tear lengthwise.