Either way will work. The plastic is the semi-spherical bearing material inside the joint. The steel ball-stud rests in it. Grease must flow around the plastic shell to get in between the steel ball and the spherical shell; a small hole probably helps distribute grease around the inside. Note that you'll either need extended-length Zerks or fabricate an extension so as the be able to tighten the Zerk into your cenered hole. Doing the tie-rod ends has the same plastic inside but doesn't need extensions.
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