On inner case necks, they should never be squeaky clean. Certain tumbling methods and cleaners are particularly bad in that respect.
On fired cases, the powder residue left in the case neck is all they need for seating. If you feel like you have to do something, a quick pass with a nylon brush and you're done.
For lubing the inner necks for sizing when using an expander... lots of stuff works well. You want to reduce the friction as much as possible so the expander doesn't pull the neck out of alignment with case as it passes up through the neck and expands it.
With a standard F.L or N.S. die, the best approach is to not excessively reduce the neck diameter in the first place.
