If the pattern includes pressing directions, I'll follow those. Many modern patterns will specify pressing open, while more traditional patterns will have pressing to the side. Pressing to the side allows seams to nest; I have fewer unmatched seams when pressing to the side. Sometimes I'll combine both in one kind of unit, for example, pressing the final seam of an hourglass unit open so it lies flatter. I like the sturdier look and feel pressing to the side gives to the back of the quilt top.