i've made tons of placemats over the years- when i use batting i generally use warm & natural ---because it's fairly flat-
i have never (pre-shrunk) my batting- as long as you quilt them i've never had any adverse effects from them coming out of the wash on a fairly regular basis
i've also made them with just a layer of flannel in the center as batting ---that i did prewash....those worked out great too.
my mother tends to prefer table runners & placemats with flannel for batting- instead of batting.