"Seam strength ... is it such an issue now, with most of us machine stitching, with modern threads/fabrics and our ability to adjust stitch length? Perhaps it was more so when the quilts were hand stitched?"
I have been sewing clothing for 40 years, and not once have I ever pressed a seam allowance to one side to make the seam stronger.
I press seams open on about 90% of my quilts.