In fact, both options will work as shields, "grounded" to VDD/VSS/GND or just keeping floating, but as charges are induced in the conductor, an electric field will be radiated from that line. If you force that material to have a steady potential and if it is tied to a net with low impedance and virtually unlimited capacitance, it will be performing a more effective shielding than just putting a metal barrier between the agressor and the victim, which depending on the case, such railing could even turn things worse, spreading the induced field to more sensitive areas of the circuit (ie, high impedance nets).