While it is definitely preferred, it is not entirely necessary, though it is certainly something that will need to be remedied with a piece of external equipment. Some turntable plinths even come with adjustable legs that can, thus, be fiddled with so that any undesirable aspects of the surface upon which the turntable is placed are combatted and remedied. If a turntable is to be played on an uneven surface without any external help, then it is almost certainly going to be knocked off kilter repeatedly, leaning this way and that as a result of the lack of balance.