I was taught by LQS not to pre-wash and not worry about straightening the grain. Most quilt peices are cut so small that being off grain will not matter when the quilt is finished. You only need to worry about straight of grain when making clothing due to fabric will not drape right if grain is off. Also many fabrics are printed crooked so if grain is straigh then pattern will be off. I would rather have straight pattern that straight grain.