In nature leo's they occurr on sandy soil/rock substrate, but straight sand should work alright. As far as basking goes, if you have the light situated where it heats a solid basking area, ther shouldn't be any need for an under tank heater. In my opinion, dorsal heat is more natual and as far as thermoregulation goes, it's what most reptiles seek. If you live in a northern region though, an undertank heater may be usefull in controlling cooler "mass temps." The room where the animal is kept may have a certain air temperature, but in cold regions the "mass temps" could be much cooler. Mass temps refer to the temperatures of the ground, walls, floors, etc. of an area. If you measure the temperature of one of these areas anywhere, it is likely to be under the air temp, so in regions where the climate is much cooler the undertank heater may be effective in raisin the mass temp of the enclosure. This is not always necessary though, as raising the enclosure off the floor/surface will raise its temp above the mass temp of the room. This can be done by sitting the enclosure on cinder blocks, 2x4's, etc. If this is not a concern though, or if you are offering 24hr basking sites, an undertank heater would not be necessary.