I remember in Home Ec many years ago we used to use two people on opposite corners of the fabric to stretch it diagonally, then again on the other corners. Teacher made us do this in the classroom before we were allowed to cut. This was for garment making. It's very important to know where the straight of grain is when sewing garments - the longer it is (a long skirt) the more important it becomes. Note: you may find garments for sale as 'seconds' that were not cut on the straight. They look normal but don't hang right. Ever had a jacket that seemed to be "pulling" on one shoulder? Straight was off.
I've never done this to fabric I am quilting with.
When cutting for quilts, the only time I worry about absolute straight is when I'm using directionals - and then it may not be the straight of grain I pay attention to if the directional isn't printed exactly to the straight of grain - and that happens quite a bit actually.