You can use just live rock with an argonite or sand substate but most people will use some live sand to seed their substrate so you get micro life population growing faster. I recomend using both you only need a little bit of live sand mixed in with your substrate so it doesnt cost much.
Also Live rock is a good idea in any tank as it provides the tank with "homes" for your micro critters that keep thee tank going .. Live rocks increses the load your tank can handle