Read the package instructions.
I bought a cotton Quilter's Dream batting- was planning to wash it - and on the BACK of the label it said: DON'T wash it - shrinkage 1% or less -
The LAQ that quilted it said it worked up fine, but when I took it out of the wrapping it was lumpy, bumpy, and wavy where it had been folded. I sprinkled it and threw it in the dryer, and it was still lumpy, bumpy, and wavy when I took it out of the dryer.
I wash Warm and Natural - but my battings are maybe ten years (or more) old, now, so things may have changed.
I like the flatter look.
Another case of "Do it YOUR way"