Is RocFall able to calculate slopes both left to right and right to left?

The direction of the slope does not matter, and models can be set up left to right or right to left.  If there is a difference between the two sets of results, make sure you check your initial velocities.  Positive values are always to the right even if the slope is from right to left.  It is possible that you may obtain slightly different values due to the random number generation used in the RocFall algorithm, but these results should be insignificant for a large sample size.