Does it matter? I'd bet some will say YES and very loudly, but I've found challenging DS on just one aspect of his writing - so say a backward 's' - is enough. Any more and it's detrimental. I don't dwell on other mistakes and give heaps and heaps of praise. Homework's a much happier time for us because I've stopped being a perfectionist and if you just concentrate on one single problem, you've more chance of them getting it right so feeling good about themselves.