Prism99

It is typically not a good idea to wash precuts of any kind because they can shrink and distort, making them difficult. Flannel shrinks and distorts even more than regular quilting cottons. You could prewash the flannel yardage but, since the precuts won't be washed, you don't really need to. See my previous post about how batting controls the amount that fabric can shrink once the layers are quilted; you simply need at least a moderate amount of quilting to keep the batting in control.

I would warn against washing the top before it is quilted. You can end up with a distorted mess from pieces of fabric shrinking at different rates and in different directions. Also, flannel ravels more than regular quilting cotton (which is why some quilters use 1/2" seams with flannel). You could end up with a *lot* of threads on the wrong side. If a top absolutely has to be washed before being quilted, the safest way to do it is to first closely baste the top to a piece of muslin or other fabric, then hand wash and lay flat to dry. I would not recommend even that for a flannel top because of flannel's horrendous tendency to shrink.

I have washed some flannels that did not shrink at all, and others that still shrank after their 3rd trip through the washer and dryer. There is no way to tell which kinds you will have in your quilt top. This is why it is much safer to quilt before washing so the batting will take over and shrink everything evenly. Fabric shrinks entirely differently when it is washed and dried on its own compared to when it is quilted to a batting and backing.