The big thing is consistency. I typically sew a bit scant and it can add up by the time a block is more than 6" (and especially by the time a quilt is finished) so I have to trim them periodically.
No one knows but us if our top is an couple of inches bigger or smaller than the directions.
I believe the scant seam is partially a hold over from pre-electric quilting before electric irons and such. At least for me I don't get a flat seam by hand sewing and finger pressing.