If you got alot of seams coming together you need to open the seam. There is just too much bulk to make it lay flat. I have always made open seams doing garments. However like on a 9 patch its easier to put it together with row 1 and row 3 going one way and the middle one going the other way. They are just easier to put together. Some times we have alot of seams to make that are close together and that would be a headache to work if you opened the seams. They tell you to press to dark side as if you could see thru a lighter fabric you see the shadow of your seam. By pressing to the dark side there is no danger of being able to see that seam on the lighter fabric.