UPDATES ON THIS MATTER:

I'm glad you got your dress working. I don't have the capability to make use of Poser's subd routine, but I would be interesting in examining the dress object that was posing problems for you, just to see if I could find anything unusual going on with it. I would need nothing other than the object file, I you wouldn't mind someone taking a look at it. (I'd delete it after I finished examining it.) But, if you don't want to share it, that's fine, too. :) There are a number of things that can cause issues with "gaps" appearing in certain components based on geometry, whether they're a figure or prop. Split vertices used to be one of the most common issues, since Poser doesn't like them. Another problem can be seen with soft-shading effects and geometries that don't have edges that are well-defined enough for Poser's soft-shading algorithm to make sense of. In that case, it tends to bork up the edges and, in seriously problematic meshes, the only way to correct it is to turn off soft-shading for that object. (If fiddling with the angle values doesn't help.) But, there are actually several soft-shading effects going on and just unchecking the object's "soft shading" box will not always take care of rendering problems if you you wish to have soft-shading on for other objects in the render. In that case, you usually have to lower the values in the soft-shading options for that object to a very low level. Material nodes sometimes have issues (or used to) generating patterns/procedural textures across geometry and shading groups. And, of course, unwelded meshe portions in a figure can be displaced unnaturally by the rigging in some areas where one would want them to follow adjoining geometry closely.

But, I only have experience with versions up to Poser Pro 2012, so I have no working knowledge of versions higher than that. There could obviously be other issues going on. :)