Speaking as a quilter who makes mostly wall quilts at this point in my journey, gravity will most likely be more of a factor for your vertically hung quilt than laundering will be. Rotating it 90° every three months would probably do a lot to prevent batting sag if you want to try that.
Sparsely quilted horizontal bed quilts are much less apt to wind up with droopy pockets of batting than than sparsely quilted vertical wall or window quilts. And when a batting specifies a 4" quilting distance, that means in any direction, so those 6"x15" open areas would actually be 4 times over the suggested density. Just wanted to point out some other things besides the laundering for you to consider.