This is a question I get from a lot of beginning quilters, "Should I pre-wash my fabrics?" I do not pre-wash for a couple of reasons. One being the look and the other being time.
First of all, I like how the quilt looks after I wash it. The fabric within the quilted design shrinks up just a touch and I think it gives the quilt an older antique look to it. The entire top, if quilted with a medium dense amount of quilting, gets a slight rippley, wrinkely, puckered look that I prefer. It doesn't mean that you have to do this; it's just my taste. Below is a picture of one of my finished quilts, with medium density stippling and the antiqued look that I love.
You could always do your own test to see what you like. Pre-wash some of your scraps, and do some practice stippling on them. Then stipple on other scraps that haven't been washed. After the quilting is done, wash it and compare to the pre-washed fabric. This will give you a clue as to what an entire quilt will look like. I used to think pre-washing would cause the fabric to fray or fall apart at the edges, but it is only minimal, if any.
Another reason I don't pre-wash, is the additional time required. I do not want to take the time to iron and refold the fabric so it's ready to use again. This is just me not wanting to do more work than is necessary. I've also heard from a few quilt shops that quality fabric doesn't tend to shrink nearly as much. I try to get the most out of every inch of fabric. I save even the smallest of scraps for paper-piecing and applique (never know when you'll need a touch of a certain color). Waste not, want not. I'd like to think that if I pre-wash, and the fabric shrinks up, then I'm missing out on a few extra inches of fabric. When you consider all of the fabric that a quilter buys in their lifetime...WOW! Fabric isn't cheap and I want to make the MOST out of what I am buying.
Now, I will end with this... I have always been lucky when it comes to washing my quilts, BUT, a major reason to pre-wash is that this will prevent colors from bleeding. Such as a dark blue or red bleeding onto an adjacent white. I would be devastated to know that I had fully completed a quilt all the way from picking out fabrics and designing, to quilting and labeling, only to have the colors bleed and there be nothing I could do. I ALWAYS throw in at least 2 shout color catchers when I wash the quilt for the first time, and I check the fabrics when I buy them for colorfastness. To do this, I place a small scrap of the fabric, usually a piece from the edge that I squared up, or the selvage with a small amount of the fabric on it, in hot water for 5 minutes. Then I check the water to see if it has been dyed, and also blot the fabric on a white rag. If no color shows up, then I know I am good to wash the fabric and not worry about it bleeding. I have not ever had a problem with colors bleeding when I do those two things.
So, the decision is up to you, I never pre-wash but if you would prefer your quilt to not look as antiqued or if you are just too worried about fabric bleeding then you can pre-wash!