It depends on the block for me. Most of the time, I press seams open, that's just how I learned to quilt, but I also know there are times that it's better for the block to lay flat that the seams needs to be on one side or the other. Blocks like the pinwheel are ones that I insist on pressing the seams open, way too many intersecting seams right in the middle of the block, the bulk and weight just feels weird, but blocks such as a nine patch, I can press to one side and I'm happy with it.