Way back in my garment sewing days I learned to press it the way it was sewn then press how you want the finished seam. The reason given as I remember, was that pressing the way it was sewn first ‘relaxed’ the tension on the stitches and smoothed the small ripples left after sewing. In addition, as QuiltnNan stated above it also buries the thread into the fabric making it easier to turn the seam.