Karen Brown of Just Get it Done Quits on YouTube has a great video on prewashing or now. https://www.youtube.com/watch?v=7f0HCDygUeI

She makes three small quilts for comparison. Imagine the difference it would make on larger quilts?

I wash fabric, for health reasons. I react to the chemicals in the fabrics. I get eczema which is very painful, so I wash the fabric.

But I found the information in Karen's video to be useful for other reasons.