The subfloor for any job is important on any job in order to receive the flooring you are putting down. It is especially important here in Texas, since we have such soft soil, and the foundation move a lot. You must ensure that any subfloor is level in preperation to receive the hardwood floors. For a slab foundation that means you must use self-leveling float to make sure everything is even. For a wood/plywood subfloor that means usually there is some sanding that needs to take place to make sure everything is even. Once your subfloor is even then it is time to install some hardwood floors!