I would definitely match the background. Any other color would detract from the hand embroidery. Even if you are doing an all over stipple the light in the dark colored sashing would not detract near as much as dark on the light colored background and fighting with your embroidery.
I think the ideal solution would be what Joellyn suggested. That you may wish to stablize the quilt by SID and only stipple in the cream and do something different in the sash.