Some quilters do this and some authors recommend it (depending on how old the publication is). There are some quilt and fabric shops that cut their fabrics this way. Tearing on the crosswise grain (selvage to selvage) is suppose to give you a straight edge. I've sone this sometimes if I'm cutting from yardage and often found it not to tear on the grain. Tearing also leaves a damaged or bruised edge. If I decide to cut my borders on the lengthwise grain to avoid piecing strips, I'll add an extra 1/2" on either side then trim those down.