If the castle nut spins when the car is moving then you don't need a cap. If the castle nut stays in the same place (like on just about every front spindle on every car you had before front wheel drive came along) then you need a cap because the outer bearing is right behind the little washer behind the nut. I have a cap on my rear hubs, even though I don't need it, because the nut is attached to the stub axle that is splined in to the hub and the hub is spinning with the stub axle. There's no bearing right behind the nut like there is up front, so there's no reason to have a cap. Here's a pic to help you visualize it:
