I'm going to have DD8 read the posts about piano practice, especially "extra stinky" and "slightly stinky"! With her ribald sense of humor, she'll have a good laugh.
I'm dealing with the same issues with both my DDs. I push DD8 for excellent work, but occasionally I let things slide. I rarely check her homework, and if she makes careless mistakes, I will simply point out -- that was careless. None of us is perfect. DD8's handwriting is awful, and she most likely will get "needs improvement" in her next report card, to which I say "So what?"
As I said on another post, music lessons are a wonderful way to stretch your child's brain, and to make your gifted child realise there's always room for improvement. My DD8 is finally realizing learning can be tough, simply because of her piano lessons.