I make it a habit to press to the dark just because there is less chance of it showing. The exception is when it would put the seams too close to another seam. An example would be if your snowballs are small, pressing towards them (even if they are the darker) would put that seam very close to the edge of the block. I almost never press seams open. I believe if can cause problems with batting and with strength. Even in an area with a lot of points, if they are all pressed in the same direction if should lay flat.