I was taught to press them open, and they always lay flatter when I do them that way. I do tend to press to one side if I'm strip piecing or doing bargello type things. The 2nd quilt I was taught to do was a Lone Star and all the seams were done open. The center was beautifully flat. So I guess my answer would be that if I'm using Jelly Rolls, I press to one side and if I'm piecing something where I don't want too much bulk, I iron them open.