If the slope and the wind favor opposite directions, one must make a decision based upon relative strength of the wind and severity of the slope. In no wind conditions, use the down slope to get airspeed and allow the ground to drop away from beneath you (otherwise, you must waste climb rate in overcoming the hill).
It may seem trivial, but I do have some doubts.
Going downhill, will the rotor RPM "catch up" with the forward speed? Too fast forward with low rotor RPM? Flapping?
The drawbacks of uphill are obvious.