I think this is situational. If a NPC asks you to get him an item, it would be pretty dumb if the quest ended without you returning to the NPC with their item. However, if the NPC simply asked you to kill a certain NM to give the NPC some peace of mind or something, and the only reward was exp and gil, it would be fine if you simply got the exp as you killed it and could find the gil in your inbox next time you went to your house.