Really don't care for on camera flash; it may be good to have for fill, but I wouldn't use it as the light, because I think it makes your 7D a very expensive disposable camera. If she's going to be in a spotlight, you can zoom it with your lens and just fill a bit so you don't wash out the spotlight, but give a bit more even lighting and a kiss of light to the eyes. Also, you may want to have a bit slower shutter speed to let in the ambient running lights; even as little blurs they can look cool.
I'm not really sure what a grand march entails, but it sounds like something a bunch (or all?) the kids do (I think we had something similar when I was in the homecoming court, but we called it something different), but the beauty of that is that, you only have one chance for your daughter, but you can probably use some of the earlier kids as practice once you get there and see what works best....hopefully she's not first! 