Pressing only to the dark, can make for bulky seams in some areas. I try to press so that at the intersections they go in opposite directions, they call this "nesting" the seams together.
Pressing them open eliminates bulk, and really helps like in OBW's (hexagon blocks)
Some blocks I press to the side and then when I put the two halves together, I press that seam open, to eliminate bulk.
Other blocks that have seams going in a circular direction, you may want to press your seams that way too.
Many patterns will give you directions now on which direction to press them :D:D:D
Using the search feature at the top of the page, typing in Pressing will bring up a LOT of topics on this :D:D:D