Learn to Make a Quilt From Start to Finish FAQ
There are pros and cons to washing or not washing. The pros to prewashing fabric are that the fabric shrinks, the sizing will be washed out, and for any fabric that is going to bleed, the dyes will come out in the prewash and not all over the quilt the first time it is washed and the fabric becomes softer.
The cons would be that you lose the stiffness, you have to iron the fabric after you take it out of the dryer, and you'll have to clean up the edges from the frayed fabric.
Pros to not washing are that the fabric keeps its body/stiffness and therefore can be easier to work with, and the colors/dyes stay bright.
The cons to not washing are that the fabric shrinks the first time it is washed and creates an "old" look to the quilt. Also, if there are any fabrics that are going to bleed, you are taking your chance the first time you wash the quilt.
I only use steam if there is a stubborn wrinkle that a dry HOT iron won't remove. Too much steam can cause the quilter/piecer to distort the block because the steam wets the fabric. So if you must use steam, use it lightly.