The dish is a simple parabola, focusing the signals from one direction to the focal point of your LNBF. If a small part of it gets dented out of shape, the signals that land on it bounce somewhere, but not to the focal point. The worst case is that you'll lose a percentage of signal quality proportional to the amount of the dish that is now out of shape.
One of my dishes still has a couple of dings in it from my original installation misadventures. It still works fine.