I have always mounted my rear RS9000's can up.
Double tube cellular gas shocks always wear-in a dead spot if allowed to sit, regardless of mounting (can up or down). People claim the dead spot is due to can up mounting, but it exists on can down mounted shocks as well. People also claim it indicates a failed shock, again it exists as a byproduct of the shock design, not a failure.
This slack spot that people test by hand working the shock only appears on the first initial movement of the shock after letting the shock sit. It's an impressive sales counter demonstration that does not apply to real (non-countertop) conditions. Continued cycling of the shock is fully dampened (like what is experienced by the shock when mounted to the vehicle).
Test the results by hand for yourself: close the shock and feel the slack on the first stroke and then continue to open and close the shock to experience that the slack is no longer present after the initial movement. Try it can up, and can down, and see if you can tell a difference.
The first cycle dead spot is due to the cellular foam breaking down and leaving a small pocket of gas in the hydraulic fluid that prevents immediate seating of the shock control valve discs, something that the high pressure monotube shocks avoid by design. The valve is fully submersed in fluid, and working properly, once the initial travel slack is consumed and the valve is wetted.
A high pressure gas charged monotube shock with the same valve assembly (and bypass assembly with a RS9000) is a better design, although with a higher price. Edelbrock offers double and mono tube shock designs with the same valve assembly to offer a choice (performance and cost). Rancho offers no choice (and a monotube would need an external bypass tube to duplicate the RS9000).
Bump is compression travel, rebound is extension travel: there is no difference in compression and rebound valving, with can up or down mounting (it's still the same compression or extension of the shock, the shock valve has no gravity sensitive components). The only shock valve design that is position sensitive is the RACOR/Edelbrock inertia valve.
Happy Trails!