Purposely creating non-planar polygons is considered extremely poor modelling practice, not just on models destined for Poser. On a model of any complexity you will definitely end up with problems. Non-planar polys are often the reason that people end up with what looks like black holes in their renders. The same goes for n-gons in Poser. I can guarantee that at some point they will cause render errors. The best solution is just not to do it.