Paintball tanks are generally made of two materials: metal (either aluminum or steel) or carbon fiber. Metal tanks are known for their durability and typically don't need to be covered, though it is always advisable. Small scuffs and scratches will not render a metal tank useless, though do be aware that dropping your tank on its threads can dent the bonnet (the metal piece with the threads) and your tank will no longer make a good seal when you screw it in.
Carbon fiber tanks are more susceptible to damage and need to be covered at all times. A bare carbon fiber tank can easily be scratched or dented which could make it unsafe to use - never use a tank that has any scratches deeper than a superficial scratch, any dents or any bare threads. I recommend using a padded barrel cover that is designed specifically to cover your tank, but something as simple as an old sock is better than nothing.