Assuming continuous lapping, unless you are concerned with crossing the lines wheel-to-wheel and want to know who crossed first, the fore/aft location should be immaterial. The sensor is measuring the time from successive passes of the transponder over the sensor loop, which should be the same whether the transponder were on the front, back, middle, or being dragged along 10 feet behind the car.
If you ever have a standing start, the furthest aft position would be best. Why? Because your car will be at its maximum possible speed when the transponder crosses the sensor line. If the transponder is mounted on the nose, then you would lose a car-length's worth of additional acceleration over a mounting on the rear bumper. This is why I advise my AX students to hold back from the start line as far as the starter will allow - every additional foot is additional acceleration time before you cross the start line.
This should not normally apply in DE and TT events, unless the first lap from a standing start or from pit road is being counted.