If shielding a humbucking guitar with hum mounted to metal plate (most are), with leads traveling through shielded cable (most are), with grounds soldered to chassis of pots (most are), the only unprotected areas are the small pickup hot leads were it emerges from the sheilded cable and terminates on the pot and the capacitor on the tone pot, which is grounded on one end. So basically, shielding only protects the small area of un-shielded pickup lead and humbucker coils if uncovered.
On a single coil, no amount of sheilding will ever make them not hum. It may keep them from humming more than normal, such as when near a Neon sign in a bar.