I soaked mine in hot water and put it in the dryer on low...must have caught on something because it came out with a few bad spots. I also prewashed the fabric in hot water. After making the bowl covers and quilting them slightly, I washed them, and still got some shrinkage. I think the best thing to do is heavily quilt the items, or just tell people they shrink and get wrinkly. It doesn't stop them from working!
I'm thinking that the only way to do heavy quilting is to use two pieces of batting, one for each fabric piece, and quilt them before sewing your darts. I'd do a basic meander, not try anything fancy. Then finish as usual. I've made three in the past two days, but haven't tried the all over quilting idea yet. It looks like a 10 or 10 1/2" square is a good size for average bowls.
If I get any that look really good, I'll post pictures!