Welcome to the Quilting Board!
When I happen to come to a seam that are pressed the same way I simply clip the seam and make them nest as they should. Be careful not to clip to through the seam
If you press seams to one side that side will have four seam allowances, not two. Pressing the seams open makes a lot of sence, unless you are hand stitching.
This life is the only one you get - enjoy it before you lose it.
With 20 pieces in a block it's gonna be downright difficult to get all the seams pressed correctly for them to nest. But I do as ghostrider does and make a careful plan before I start. I am a handquilter and really don't want to quilt through seam allowances if I can help it. Besides, pressing 1/4" seams open would make me crazy! Occasionally it is worth the effort if you have many points coming together in the center of a block. Then I might press one or two seams open. Generally you can find a way to get "most" of them working though. It's just important to plan ahead.