If you need to quilt something quickly on your DM, think about 'shadow' quilting--stitch just a bit away from in the ditch. I have found that a little variation from the exact path is much less noticeable than when stitching directly in the ditch.
It is not a hard and fast rule to always iron to the dark. If I am making HST's for flying geese or star points, I will iron one to the dark and one to the background. That way, they nest nicely and I get a smooth join. If I am making two colour 4 patches and want to swirl the centers, two of those patches will have seams ironed to the light.
Tartan, if you are seeing thread and batting, perhaps you need to adjust your thread tension. There should not be that much of a gap in a pressed open sewn seam.