It is best to press to one side. That being said if you are now joining blocks and find that all your seams are going the same way you can still take about half the seam and press to the opposite side so they still lock together. I do this all the time for the same reasons. It seams no matter how hard you try you'll end up with seams going the same directions and they don't lock or but up against each other as well. By pressing them about half way down the seam in the opposite direction they'll still lay flat. I do not recommend pressing them open because first of all they are stronger closed, 2nd if you are going to do SITD you won't have the ditch if they are open. Try this and see how it works.