RobertH,
Nurbs to Poly has big disadvantage you can not modify converted objects. If it is not critical when you just Model something but it is critical when you make Concept.
Practically all stuff that you can make after converting Poly to Nurbs you can make in Poly. So my experience show me that it is better to not convert poly to Nurbs. If yo make some parts in Nurbs you can merge them with .obj.
In Internet there are a lot of video showing how converting Poly to Nurbs is great but on practice I find it useful not so often.