I fear tobspr and I are giving you conflicting advice. :-/ In my opinion, rendering back faces is the best way to avoid artifacts, and creating extra geometry to patch up corners. Rendering front faces (or using double-sided rendering for that matter) will result in shadow acne, which can be worked around using a depth offset (nodePath.setDepthOffset(1)), but this will cause peter panning artifacts that will also cause light to bleed through corners like that one.
You can download development builds of 1.10 on the download page.