Tearing fabric is not commonly done now days since the fabric making/weaving process has become much more refined and quality fabrics usually have a pretty straight grain.
You only tear 100% cotton, some silks and some velvets as it will give you the true grain of the fabric. That makes it easier to square up your yardage for anything you are making. A distorted grain can ruin all your hard work in making the perfect item be it a quilt, bag, dress or potholder. If you are buying fabric of questionable quality, ask the clerk to rip it--if it doesn't tear, it isn't 100% cotton that means you may not be able to straighten the grain if it's crooked. If you really love it (the fabric), you may need to change your plans for it.