So, based on what y'all have written, I'm thinking of doing it like this:
-Putting something like fusible web on the back of the sky (I don't want any stitching or clouds on the sky).
-Quilting the batting to the backing on the sky portion
-Then basting the layers together, and quilting the rest of the wall hanging
As cad_queen_2000 pointed out, that would take care of any problems with the batting, and keep the sky smooth and flat. I'll have to do some experimenting, but that will be my starting point.
Please let me know if you have some more advice...I love this board!