We just did this. The proctors sat her right next to the window and door to the office in case she needed help and they were very kind and sweet with her. We did the practice test questions online before hand and there were also some screenshots of the test on the CTY site.
That said, my daughter has some trouble with the math portion of the exam. It seems that there was a timer running for the instruction reading portion of the test and she got nervous and thought it was her remaining time. She asked the proctor and they reassured her, but it was still a lot more stressful than the first half. Result is that she passed the first (passed meaning qualified for camps and classes) and not the second. Of course she's also weaker in math, but the combination wasn't a good thing.
I like the idea of computer practice, but until you've done a timed test in a testing center, it's going to feel unfamiliar. DD wants to redo the test at some point (we missed the registration window for camps anyway -- not to mention needing time to plan for the expense) so it's not a big deal. Knowing she could made her feel a lot better. I told her to think of this one as practice for taking tests and that's probably a good way to look at it.
