I rarely press seams open. I actually like the little hump, if you are trying to stitch in the ditch, that little hump makes it easier to do as I like to rub my needle on the hump like just driving your car wheel along the curb. Yes, the thread and fold do take up a tiny bit of space. That is why I measure the final size of the block and see if I need to use a slightly bigger or smaller seam.