I have never heard of cassocks/anderi being worn by lay people in the nave or narthex area, but often (at least in the GOA) lay people who are serving at the altar in some capacity will 'borrow' an anderi. The lay people will have some sort of blessing to function at the altar---just so the detractors know we don't allow just anyone to wander into the Holy Place and do things---and before I was ordained I would do just that when I helped out near the altar.
Again, my answer is specific to the Greek Orthodox Archdiocese in the United States, but I cannot imagine a reader, subdeacon, or acolyte serving in the altar area in their street clothes. I am virtually certain that wearing an anderi or acolyte's robes is a requirement, not 'generally accepted practice' in the GOA. Other jurisdictions may have a different answer, though.