If you feel more comfortable in flats, then perform in flats. If you prefer heels, then play in heels.
Some of my female friends prefer to perform in heels because the pianos are on a wheeling platform and so the pedals are a tad higher as a result, and the heel makes it feel a little closer to normal. Other female friends of mine think it feels too unstable. It's all a matter of preference. Nobody is going to be paying attention to your footwear unless you walk on wearing flip flops or sneakers.