I have always used the selvedge edges as a benchmark of straightness. When I did a lot of garment sewing I measured in from the selvedge to align the grain arrow on the pattern piece. In quilting I use it to be one of my marking points to straighten fabric up. I line up my selvedge edges together and move them right or left until I get a clean fold. The edge that has been cut at the fabric store is almost always skewed when I do this. I lay my ruler so that one of the lines on my ruler goes along the selvedge and the ruler itself is perpendicular to the selvedge and I then cut along the ruler to square up the end of my fabric. It is that clean cut that I then use to measure my other pieces from. If I have a large piece of fabric that I'm cutting, for a backing for example, I tear the fabric rather than cut. The tear will go along the grain of the fabric. I always cut off the selvedge in a piece that is going into a quilt but I leave the selvedge on the fabric as long as I can so I have a reference point.