Basically, if you are modelling a head. Create a nurbs sphere, adjust the cv's to get the core shape against your reference, maybe add a few more isoparms (edges for nurbs) if necessary, then go to modify>convert>nurbs-polys, and adjust the options to suit. I tend to keep it low poly and refine the head in polygon mode. Though you will have to delete the nurbs model you had as during the conversion this creates a separate piece of geometry rather than replacing the old or converting it as its name suggests.

Hope this helps