API 598 (section 4.4.1) infers that the packing should not be greased because it is a "sealing surface."  Also, MSSP-61 (4.1.2) also says that seat surfaces should be clean.  It could be inferred that the packing should also be clean because it creates a seal on the stem of the butterfle valve.  Some people in our company interpret this as strictly the seat of the valve and therefore it is perfectly fine to grease the packing to get the valve to pass a pressure test.
Assemble the valve according to the manufacturer's assembly/service procedure.  Unless you have a lubricated plug valve there should be no grease(Sealant) inside the valve.
On most of the valves I have seen:
TFE packings are not usually lubricated.  
Grafoil packings are put together with a dab of never-seez
Certainly the old days of asbestos packing and a grease injector on the side of the bonnet are gone.

MORE NEWS