Pretty basic question, and I'm surprised that I can't give you an authoritative answer. The shop manual says to "pack" the hubs with grease, and I've always taken that literally--fill the volume with grease and also work it well into the individual bearings. If you grease only the bearing, the motion of the bearing will force out the grease, so filling the volume keeps grease in the bearing.
Or, maybe I'm all wet. I'd be interested to hear what others have to say.