Placement will not affect accuracy (the inner bit of the wheel rotates as many times per mile as the outside) BUT here are some guidelines:

1) Putting the magnet and sensor closer to the hub will in general be more accurate because the magnet will be moving past the sensor more slowly, so you'll be less likely to skip counts when the wheel is turning fast.
2) It's possible to mount some magnets in such a way that both the north and the south pole trigger the sensor, causing double counts, sometimes only at lower speeds, and this can also cause skips at higher speeds.