The Piece-o-Cake lady has some nice reasoning for pre-washing:
http://pieceocake.typepad.com/piece-o-cake-blog/2013/01/yes-you-really-do-need-to-wash-your-fabric.html
One point from the article is that when you wash the sizing out, it makes piecing easier because the fabric doesn't slide around. You don't have to pin as much. And I'm with her. I don't use any sizing on the fabric while piecing. I do sometimes starch before quilting because I find that over the course of a day or so the crease from ironing goes away and my borders start to flap around.