It is sometimes nice to cut sashing and borders on the lengthwise grain if you do so at the beginning of the project. However, it is customary to cut binding on the crosswise grain or the bias grain because you can easily control the stretch. If you join these pieces with a bias seam they are almost undetectable. Someone gave a link to a chart for estimating how much you need to bind various sized quilts. I tuck it in the back of my wallet just for reference. You must be careful of the 'knowledge' clerks in fabric stores have. Much of the time it is very little.