I think we mostly began pressing the seams to the side in the days when we started frequently using polyester batting, because these often "bearded" through the seams when we quilted.
Bearding is when tiny little tufts of the batting comes to the surface of the quilt top and can be seen like little lint clusters on the surface of the quilt. Because the tufts are usually still attached to the batting in the middle of the quilt, it's hard to just pull or brush them off; they usually are best removed by cutting them at their base.
Jan in VA