That depends on the light levels your plants are being grown under. The higher the light levels, the higher the plant's need for carbon. So if atmospheric C02 is enough for your plants at the light level they are receiving then you would not need to supplement C02, provided there is enough fresh air exchange in your grow room to replenish consumed C02 by your plants. On the other hand, if your light level is higher than what can be supported by atmospheric C02 levels for optimal plant growth, then you would need to raise the C02 level higher. This will prevent your plant's growth from being carbon limited, which would cause the extra light to just be wasted, as plants need proper ratios of light and carbon (along with other factors) for optimal plant growth rate.