Here's what I've been told. You want the 'dirty' side up because it is needled from that side. If you have any doubt, take a needle and see if it is easier to push a needle through on one side or the other.
The other thing I've found is that if you have the wrong side up, especially with W&W or W&N is that you can sometimes get pokies of batting pushing through the back. They will slough off in time, but if your back is black, it will look awful! It's not that there is anything wrong with the batting, it's a chemical reaction between the chemicals in the fabric and the batting.
HTH!