Like others have mentioned, most of the time you sew and subcut is because you'd be dealing with small pieces to start with and that is difficult doing on a machine, but it's not difficult doing by hand. I'd be another that would consider reverse engineering and sub cutting before sewing.

When I'm machine sewing blocks with small pieces, I usually cut them larger and sew, and then trim down. There is no way I'd try to sew a 1/2" finished HST on machine if I cut them at the correct measure to start with. But if I was hand sewing, I most certainly would cut them the correct size to begin with and not plan on trimming down after sewing.

I handpiece all of the time and you have to plan ahead a little. What I do is mark my 1/4" seam with a fine mechanical pencil. Then I make a tiny mark at each corner 1/4" in from the edge. Then when I hand piece I insert my knotted thread a 1/16" of an inch or so before the 1/4" from the edge mark. Then I stitch backwards to the edge, and backstitch back to the mark. Then I stitch the seam and backstitch at the 1/4" mark on the other end, stitch to the end, backstitch, and finish a little over the mark. It's hard to explain, but I never have a problem with cutting my stitches and having the pieces compromised. I always trim to square.

I find that marking your seam allowances is very important. I haven't had a problem using a light pencil mark. I also square as I piece so I rarely have issues with unmatched points.