If you have an instrument with a metal bridge plate, it would be good practice to ground that plate rather than leave it floating. Because the bridge is a big chunk of metal near the pickups and their wires, which are high-impedance and thus prone to pickup. If the bridge is floating it will tend to help to couple the ambient 60Hz field to the pickups and wires. On the other hand, if it is grounded it will tend to act as a screen, shielding the pickups and their wires from the ambient 60Hz field.