I prefer to press to the dark side or the side that the seam naturally wants to fold when the two colors are quite similar. This is likely just habit. But, occasionally, I will come across a pattern the insists pressing open will give a better end result. In that case, open it is. The leader of my quilting group insists that open is always best but she gives us the choice and will indicate when doing our sew-a-long if open is better.
