I have pre-washed Warm and Natural as well as other cotton battings many times. I soak for a few hours in the washer with warm water and little or no agitation. Instead of putting it in the dryer, I lay it out on the tile floor and let it air dry. The advantage to the tile floor is that I can line the flat batt along the grout lines to keep it square.
I am one who washes all cotton: batts and fabric - before using.