At my high school tryouts, cheer and tumbling are part of the scores! You don't need tumbling to make the team (in most cases) but it would definitely help. As for balancing as a flyer, a lot of flyers tell me that if your bases are good enough, it's easier balancing in the air than on the ground. Nevertheless, a useful object for balancing while getting used to being off the ground is, believe it or not, a chair. Although it's not exactly as high as you would be in a stunt, it is better than being on the ground. Also, be sure to stretch, and go to open gyms if you can and sign up for a tumbling class if you are not already in one. Good luck at your tryouts!