I do not tear my fabric to "straighten" it. I press the fabric first then I hold the 2 selvege edges together and let the fabric hang. Then I move the 2 selvege edges back and forth until the fabric hangs straight with no "swoosh" or bend. Then I lay the fabric on my cutting mat with the fold away from me, smooth the fabric and fold the selveges to the fold and hold up to make sure there is no swoosh. Then I clean cut one side. I learned this method a long time ago from Mary Ellen Hopkins and it has rarely disappointed me. I do not prewash either. One exception-when I want lengthwise grain pieces for borders I do tear them.