ENERGY STAR is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy helping us all save money and protect the environment through energy efficient products and practices. Energy Star says "covering your condenser unit in the winter is a good idea. It will help prevent dirt, leaves, and other debris from getting into the condenser fan and the coils. It might help extend the life of the condenser unit or just make it easier to clean in the spring."
More information regarding Energy Star Recommendations