The first time I bought batting (80%/20% low-loft cotton batting) at my local quilt shop, I asked if I should pre-wash it. The quilt-shop lady asked,
"Do you pre-wash your fabric?"
I answered, "Yes, I do."
"Then you should also pre-wash your batting."
So that's what I do.
I like the fact that with pre-washing, once it's quilted, and I've had it bunched up in my lap while sewing on the binding, that a "brand new" is already all soft and 'drapey' and cosy-looking when I gift it - but I mostly do lap and throw-sized quilts. I can see wanting a flatter result if you're making bed quilts.
Jan