Yup. But not for the reason you think. The main problem with fender pickguards is not EMI (hum), but rather discharge of static electricity. If you're like me and you tend to touch the pickguard when playing, then some static electricity builds up on the 'guard. After a time, it discharges, right into those pickup wires that are handily located right below the surface. You get an annoying crackle when you are playing. If you put some shielding on the underside of the pickguard where the wires run, then ground it, then the built up static electricity discharges into ground rather than the hot leads of your pickups. Crackle gone. BTW, the shielding can be anything from copper to aluminum foil, just as long as it does not move around and is grounded.