Leos soak in heat from their BELLIES, not from above, so an under tank heater is best. The only way they can get hurt from one is if you do NOT have any substrate and they are on the heated glass, or your heater malfunctions which has been known to happen but it doesn't happen often at all. BTW, use towels, tile, paper towels, or newspaper(anything not dangerous lol) as your substrate. You can definitely use an overhead bulb for heating but I would definitely not keep out the under tank heater. You just have to play with it(it's good to get all the cage requirements BEFORE you get your animal) and move the tank around to get the right temperatures. It all depends on tank size, size of UTH, wattage of bulb, etc. Sometimes you need both a bulb and an under tank heater, sometimes you only need an under tank heater.
Hope this helps