How are the cables run over the top? is the top of the joist notched out or is the subfloor notched? Either way, the risk is that someone driving a nail or screw from above could puncture the wire.
At any rate, drill your holes slightly above the center of the joist. This puts the hole in the area of the joist that is under compressive stress, and away from the center which is an area of maximum horizontal shear stress.