There is no problem with mixing nurbs and polys in a single model.
However, you must consider how the model is to be used. Some applications like 3D game engines do not support certain geometry types. So in this case nurbs or subds could be a problem.
In general if you are building the model for yourself use whatever geometry type makes the most sense. Nurbs are great for tubes and wires and have the advantage of being easily textured (no need to layout UV's).
As a primary geom type though they have fallen out of favor and polygons and subd's are now more popular as they do not require patch topology and avoid the need to worry about seam tangency which can cause trouble when animating.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675