when i was young, you could pull the fabric on te bias and straighten it, i it weren't square.....they always tore it.
with poly and blends, i don't try to straighten. i like it cut square, as that is how i will be cutting it for my quilt blocks.
the frayed edge makes you loose some fabric. i hate tearing.
after its been torn, line up the selvages and see how uneven the sides are....some fabrics are straighter than others. if there is a line in the fabric and you can't straighten it, you won't be able to us it for a border and have your border pattern be 'square'