i never wash my batting- but always my fabrics. it removes sizing, oils, grim, smells (lots of people fondle bolts of fabric before you buy it)
if i have a bleeder i know right away- if i have a (sub-standard) fabric that frays or separates- i know; i just feel better about handling it once it's been laundered & is ready for what ever i want to do with it. I generally purchase bolts of batting- or packaged ones that have not been (fondled) by the general public before i get them- so i wait until the quilt if finished/quilted & bound- then launder the finished product- that's when my batting gets washed.