Tartan is correct...as a longarm quilter I have found that if there are more than 4 fabrics at an intersection, it creates a lump and pressing open those seams helps to spread out the thick lump of fabric! Other than that, there is no "rule", so press in a way that works best for you. If I am stitching in the ditch, it is easier to hit that ditch with the fabric pressed to the dark - for some reason the lighter thread hides in the ditch more easily for me. But ditch work is really usually only done on a DSM if you are doing that all over or for show quilts when it is done on a longarm (expensive because it takes some time to slow down and hit that ditch precisely!).