I know for me I always press my seams to the darker side of the pattern. But when I put paper pieced blocks together that have a bunch of seams all at the same intersection then I press the seam open. I guess it really depends on the project. Case in point, I made this one block and it had a bunch of seams in the intersections and I tried to get the dumb thing to press flat but without success, so I took them out and pressed the seams open and then twisted the center seams and presto! A flat block. So in answer to your question I guess it completely depends on your block or quilt. I would also probably talk to your long armer and see if she or he prefers the seams pressed open or to the dark. Hope that helps.
Jennifer