I always tear my borders the length of the fabric (so I don't have to piece wider pieces) and have never had any problems. I usually add a half inch to the size I need, snip an inch or so, and rip away. If I want a 6" border I rip it at 6.5" and then carefully fold it so I can use my 6x24" ruler to quickly even it up to 6 inches. Works so much better for me than trying to cut out a long piece the length of fabric.