I always tear anthing that is longer than my longest ruler. You can make the strip just a smidge bigger than you need so that you can use a more generous seam to make up for any pulls in the fabric. Using the straight of grain is just as important in quilting as it is in garment sewing. If you have seen a quilt that has "wavy" borders it is because they were not cut straight or on grain. If you are doing a binding that has curves or scallops etc. You should cut your binding fabric on the diagonal or at least the crossgrain. Both allow the fabric to stretch as needed. For solid borders the straight of grain is the only way to go. If you have ever finished off a bolt of fabric you will note that the last edge may be really crooked sometimes almost diagonal. That is because they stretch the possilby still damp fabric as they roll it on the bolt at the factory. That is one good reason to prewash your fabric. It helps return it to it's proper shape.