Quote - ... vertex normals are important and need to be exported to the .obj.

Actually, no - Poser ignores normals within the OBJ file (I don't know if D|S does too). Poser determines which way a facet is facing by examining the "winding order" - that is, the order in which the vertices are defined in the file. If they appear in anticlockwise order, then the facet is facing you. Fortunately you don't have to worry about this, it's all taken care of if you have the normals right in Blender, but you don't need to export them.

Quote - I'm playing with a fairly simple morphing chess piece at present, and I've noticed that Blender, Poser and DAZ Studio all seem to render the same .obj slightly differently. I'm fairly sure it's to do with crease angles and shading groups. Any comments?

It's true. Poser tends to ignore a lot of the stuff that can be present in the OBJ file format (see normals discussion above), such as material definitions and smoothing. These things are taken care of in the Poser library file (PP2 or whatever).