Recently a customer asked this very question…
Earlier in my sewing career, we prewashed EVERYTHING. But things certainly have changed. Years ago, manufacturers of batting recommended prewashing. Sometimes the finished batting had small cotton seeds scattered throughout and the oil in these seeds could stain your fabrics. With shrinkage and seeds, I think that was the reason why quilters prewashed their batting.
I did some research, and this is the quick answer I can stand behind. You CAN prewash most battings – but it is NOT necessary. Today’s batting is constructed to resist shrink or shrink minimally. Modern cotton batting shrinks anywhere from 1-3%, which is just fine by me. Besides the wrinkles you get with the shrinkage, hides the not so perfect stitches when I hand stitch my binding to the back.
Why would I prewash batting? If you reuse batting, I suggest a delicate wash cycle. Hand wash if you have a washer with an agitator. Of course, if you spill something on it, it has been stored somewhere where it is smelly, or you are unsure of what it is made of please prewash.
New batting that has been stored in a dry, clean area will not need to be washed. If you receive batting from another quilter and are unsure the condition it had been stored, I would recommend a quick wash.
All this taken into consideration, under most conditions I would NOT prewash my batting.