I started sewing clothing long before I began quilting, and most of the time seams are pressed open. When I started quilting, I was taught to press to the dark, but I found I liked the results better when I press open. I seem to be able to match seams better when they're open then when they're pressed in opposite directions. I try to press open as much as possible unless it'll show through on white fabric, and then I may trim the darker seam allowance a hair so it doesn't show. I feel that the quilting will "hold everything together", so I'm not too worried about weaker seams.