Do I really need to skim?

A builder told me that there is no need to skim over a new plasterboarded ceiling. He said that if I tape and easy fill the taper edged joints it will be good enough to paint without plastering. Is this the case?

How do I know what length of plasterboard screws will I need to use to attach the boards to the original ceiling above?

You can do either, tape and skim, or tape and fill the joints/screw heads then sand down when dry!