When AC is on the cold air is released from the front vents and hot air is sucking into the AC through the upper vents. The hot air in the room always go up and cooler air will settle downwards as it's the physics. At this point if a ceiling fan is there, then it may push hot air down which result in inefficient working of hot air intake of AC indoor unit. So ceiling fan or wall fan is generally not recommended. But if it is very hot climate use of ceiling fan helps in circulating air so that efficiency of AC should increase. In this case fan with air conditioner is recommended. In spite of all these when a room gets cool a fan rotating at a mild speed can give a pleasant feel. ( assuming it's a split AC. If it's a window ac then the story is entirely different )