My mother always prewashed everything, so that was my thought when I started to quilt. Then I joined this board and read about lots of folks who never prewashed, so I decided to measure before & after to see if pre-washing made a difference. The first couple of times nothing seemed to change. A yard in the wash, a yard out of the dryer. I was just thinking about abandoning my prewashing, when I decided to also measure the WOF. I was amazed. There was very little to no shrinkage along the length, but ALWAYS across the width. Sometimes more than 2". (Except batiks... and they bleed.)
As far as before & after quilting I don't make anything a specific size, so I've never paid attention.