I always prewash bright or dark colors , never prewash pastels when making a quilt. If making clothing, I always prewash if it will be washed after wearing. and, I open fabric . I think that allows me to see if it was on the bolt off grain. Back in Home Ec class in the 50's we always had to do that, and then two of us would grab diagonal corners and pull to get the grain straight. It
sure saved things from being wonky later!