"Let's say that those individual walls are only six polys, the minimum needed for a square. If that is the case, then you are not going to be able to apply smooth shading effects to them without Poser distorting them in renders."

I think you mean a cube, because only 1 quad (two triangles) is required for a square (single sided).  If you bevel the edges of a cube, then Poser should shade it appropriately, even if it doesn't have interior segments.  "Splitting" vertices is a very poor solution, for the very reasons you mentioned, plus it might not be good for rendering with path tracing engines, like Luxrender, or iRay, because it could produce artifacts or light leaking.  I hope people aren't still doing that with their models these days.

To be honest, modelling for Poser shouldn't be any different than modelling for any software application today, as long as you're using logical, solid construction techniques, with good topology.  In the past, Poser modellers were doing all sorts of hacks and cheats to their models, to cut down on polycount, and make the models work with the wonky Poser smoothing.  However, that's mostly all changed now, and if the model is created with quality, logical modelling techniques and good geometry that makes sense, then it should render just fine in Poser as it does in most other applications.

For example, never create models with long, thin polygons.  They won't render well in Poser, or any other application really.  Long cylinders that don't have any horizontal edgeloop segments are going to render badly  in most applications, and suffer from the dreaded "blow up" balloon smoothing effect in Poser.  Always bevel your edges for more realism, and add at least one horizontal edge loop segment to any cylinder object, to help hold it's shape.

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Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.