I overcast the ends of my fabrics before washing them - I use either a narrow zigzag on the regular machine or overcast the raw edges with my serger. I set the stitch length longer to minimize the thread used. It really cuts down on the stray threads and fraying that would occur during the washing and drying processes. Yes, I know it's another step away from the really fun part, but I think it's worth it.
To satisfy my curiousity, I measured fabrics from WalMart and from the Quilt Shop before and after washing.
I soak my fabrics in hot water (I could still put my hand in it quickly without getting scalded, but it was hotter than comfortable) for a half hour (or more) and then wash it gently in cold water. I dry the fabrics on a permanent press heat setting.
Just about ALL the fabrics shrank some. Some shrank a LOT more than others, to be sure.
Some fabrics from the quilt shop have also had a lot of excess dye in them. I had to rinse them a couple of times to get the water clearer.
I wash ALL fabric I'm going to put into a quilt. The ones I make are meant to be used and it's almost a given that they will need to be washed at some point.
The first time I wet the fabrics, I keep the colors separated. That way if there is a "problem child" in the group, I don't really ruin anything. If/when I see a lot of color in the water, I try to isolate the offending piece and see if I can get it to behave.
I had bought a piece of orange background Halloween fabric from WalMart last October and could NOT get it to quit bleeding. Much to my amazement, I was able to return it. The water still looked like orange KoolAid after over 20 changes of water. I consider that defective.
Also, to my amazement, I told one store about it, and the manager (I don't know if it was the store manager or the department manager) told the clerk to put it back out for sale. Sheesh!
I have about eight WalMarts within 20 miles of my home in Minnesota.