Umm... think about it. Doesn't matter where the magnet is, it still goes past the same amount of times - ie once per revolution of the wheel. The computer counts the number of times it goes past in a given timespan (the frequency), multiplies that by the number you programmed in (which equates to the diameter of the tyre, ie the distance travelled for one wheel revolution) and works out from that how fast you're going.
The only difference position makes is to the speed the magnet itself has to travel in order to make a complete revolution – closer to the hub it will be travelling slower, closer to the rim, it'll be travelling faster. But the sensor doesn't care, because it's not counting magnet speed, it's counting the frequency at which it passes.
There may be an argument to say closer to the hub is better as the magnet's moving more slowly so the sensor has more time to get a good strong count from it. But then the closer to the hub, the further the signal from the sensor has to go to the wrist unit, so you might get a problem there instead...