Being a National Merit Commended Scholar (I was that) or semi/finalist is nice but really, in the big picture, doesn't mean THAT much. I really wouldn't put so much pressure/emphasis on it. Just be supportive and wish her well.
We can't really control our kids as they get older, frankly. If she rocks the test, great; if she doesn't, well, you will still love her! Things have a way of working out, I figure. Just encourage her to do her best and leave it at that. you can't take the test for her, LOL.