I'd have no problem riding backwards down a slope with careful braking, the bike is superbly balanced even at low speeds. Just taking a little care, going slow and maintaining balance... assuming we're talking straight down the slope, not at an angle, at an angle it might get a bit tricky.
But I'd get a bike dolly instead in that environment (or turntable). A bike dolly might be better since it allows you to shift the bike(s) around in the garage, push them up against a wall or whatnot to make space. Leave it on the floor, ride on it when you get home, spin the bike in place and then ride out when you leave.
http://www.discountramps.com/motorcycle-dollies/c/2125/ - I'd probably get this for a Rocket
http://www.discountramps.com/motorcycle-dolly/p/CRUISER-DOLLY/