Well, usually I press toward the dark fabric. If I can't do that, I check after it is all pieced to make sure the dark doesn't "shadow" through the light fabric. If it does, I carefully trim a little of the dark seam allowance so it is a tiny bit shorter than the light seam allowance.
Some quilts just need the seams pressed open, like the One Block Wonder. There would just be too much bulk in the center of the hexagons if you didn't. Some people just press them open because they like to.