It's really not the thick/thin issue, it's the natural material will shrink issue.
There's no reason to wash 100% poly batting, as it will not shrink.
Natural fibers, cotton, wool, silk will shrink when washed. If you pre wash the batting, then you don't get an old fashioned crinkly look to your quilt because it won't shrink. If you pre shrink your batting, you also want to pre shrink your fabric. If you pre shrink everything, the quilt will remain closer to what it looks like after you're done quilting, but before you wash it.
So it's all about the final look of the quilt.