RS-485 shield and ground wiring practice
This is a huge topic and one can not give an answer without knowing more about the system setup and the noise sources. 

In general: Twisting the cables is good against inductive coupling (of magnetic field), whereas shielding the cables is good against electric coupling (of electric field). 

But this holds only true if the system is designed in a way that the shielded currents can not harm the signal link.