Actually, high-stress garment seams are usually flat-felled or triple-stitched, others are serged, top stitched through seam allowances... even dressmakers at home, sewing for their personal use, generally finish seams for their function. I would never make a sleeve or center back/crotch seam on pants, for example, with just one line of sewing and then press it open! I might do it on a shoulder seam, with the collar seam at one end and the sleeve seam on the other.

i typically triple stitch garments and handbags and then i would press the seams open.

I think that traditionally hand-pieced seams were pressed to one side to strengthen the seam. This convention was continued when quilters started machine piecing; however, it's not really necessary with machine-pieced seams because they are generally stronger than hand-stitched seams. Many award-winning quilters press their seams open to achieve more precise piecing results and to make hand quilting easier.

"Bearding" of batting through seams pressed open was probably more often a problem in the past. It might still be a problem today with some polyester and wool batts, but I think for the most part modern processing of batting has cut down a lot on this problem. Cotton batts usually do not beard at all.

I would say it depends on the design I am making. If you press to one side and then the next row the opposite way, the seams nestle up and you get a good match up. But, on some blocks that intersect heavily in the middle, you have to press them open to have a nice flat center. Just my personal perference, I am sure. I have heard it taught both ways. Today's machine piecing are a good strong seam, and pressing it open is acceptable.