I always iron my seams to one side as I think it's easier to match them than if they are pressed open. I made one pattern that said to press them open, but I had problems with matching and didn't think it reduced bulk.
If you would like to try, I would open the seam and use a wooden iron or your finger to open the seam, then iron so you are not trying to open the seam 1 or 2 inches in advance of the iron. You can also try using a bamboo skewer.