What he said; towards the hub will cause it to skip less, but that's actually usually not a problem. Apart from the sensor possibly missing a beat because the magnet is going by it too fast, it doesn't matter, the computer will measure the same no matter where the sensor is placed. Unless you have some really weird space-alien technology wheels, the center still turns the same number of times per mile as the outside.

Accuracy is dependent on how well you adjust it. You DID read the owner's manual, right? The part where it tells you to do a roll-out test to determine the number of millimeters per revolution of the wheel, and input that value into the computer in setup mode?