In my learn to quilt class 2 years ago it was strongly stressed about the importance of pressing your seams to the side. The reason was for stronger seams and for nesting seams easily. I happily went along with that but doing a lot of muttering when I pressed in the wrong direction and ended up with lumpy seams. However, I am now seeing the benefit of pressing open seams. For example, I am doing hexagon blocks using this tute from Lee at Freshly Pieced. I pressed the seams open and the results are great. The block lays so flat with a bit of help from Fresh Press. And maybe the Aurifil thread I just started using helps as well.