I run the dust shields on my '77 F150, but don't run them on my EB, so I would say that the dust shields are optional. Since they're sandwiched between the spindle and caliper bracket, it technically moves the spindle/rotor/hub inward by the thickness of the dust shield (18 gauge = .048"), but the caliper mount can accommodate it and it hasn't caused any problems or issues that I've seen.
FWIW, I just went through my '77 F150 HP D44 knuckles this weekend to install new axle u-joints, wheel bearings and brakes, and the dust shields were packed full of mud from the last excursion. On the up side, the spindle bearings were in remarkably good shape for having been submerged in super soupy mud for an extended day of wheeling the last time out.