I was always told to press to one side. The reasons were: 1. Stronger seams. (Which isn't a problem with today's machines.) 2. Makes it impossible to see the batting through the seams and keeps the batting from exitting the quilt.
Hold your pressed open seams up to the window. If you see light showing through the seam, you may see the batting through it, also. This could become more visible with repeated washings.
In this case, it's a wall hanging, so little or no washing involved and shouldn't be a problem.