The curve on this test is very easy. With an 800, you are only the 93%tile or something like that. So, I would wait until your kid is fairly confident to get an 800. Otherwise, a couple more mistakes will send you down 750 really fast.
The reasons my daughter took it without any preparation are: 1, it is required by one of the schools she is applying, 2) this is the last chance, 3) I could not make her do any study at all.
Remember, SAT will send ALL your results when you send scores to a school. You can't just send Math or Bio or whatever. Every scores in every subject will be sent.
Don't take any unless you know you could get decent scores.