Ripping on the crosswise grain (selvage to selvage) as suggested by Maride, will stretch the fabric edges and is not recommended by most professionals. Crosswise grain is much less stable than lengthwise grain. If your fabric is folded on the true straight of grain, you will not get a dogleg in your cut.
Ripping on the lengthwise grain is accurate and is the easiest way to prepare long borders for quilts. Scared to try it? Try tearing off just the selvage to see how it works...and feels!