We signed ds11 up last year when he was in 5th grade. We did it for one reason: it gave him experience with a test for which he wasn't expected to know most of the answers, which was helpful to get him used to letting go of the idea of perfection--he could do poorly on it and it wouldn't be "his fault" but simply a test that was so far above him. Of course, after we received the results we realized we had underestimated him--by a lot--which led to him getting an IQ test done by the school. Which led to...
It's a good tool, if you need it, at a reasonable price and I've read somewhere that as children age, these above level tests become more of an accurate gauge of ability than IQ tests.
You could wait a year or two and see if your dd's situation changes...