It shouldn't ever hurt, but it may not be useful - see, Poser polygons have two sides.  One side only is considered the "normal", or the useful side.  Depending on the model, it can happen that the normal is facing away from the camera during rendering, and that polygon (and/or other polygons like it) may not render correctly or at all.  Enabling Normals_Forward forces the renderer to consider whichever side of the polygon that is facing the camera at render time to be treated as the normal, so it should make a backwards-facing polygon render correctly anyway.

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