Mounting a shock upside down offers a couple advantages. First, it lowers unsprung weight of the suspension. Second, moving the shock body up higher moves the shock fluid away from the brake system, so it runs cooler. Three, ease of adjusting the shock. Doubtful that the unsprung weight is affected that much. Ease of adjusting the coilover unit likely plays a much more important role in how you want to mount the shock.
Most I have seen run the front shocks right side up and rear shocks upside down.
Steve