Some fabrics barely shrink at all and others scrunch up like that Shrinky Dinks plastic when you put it in the oven.
I don't pre-wash, but I do saturate the fabric with diluted Best Press before I press it - you can watch it puckering up and getting smaller, sometimes.
You don't want that happening after you've assembled your quilt - especially when the fabrics differ in shrinkage. Yikes - what a mess that could make of your careful piecing. :(