A little tidbit from
Sheldon Brown's invaluable website:
With one-magnet cyclecomputers, you usually have a choice of several places on the wheel to attach the spoke magnet, depending on the spoke pattern of the wheel. Generally, it is best to mount the magnet as close in toward the hub as possible. The closer in you mount it, the more slowly it will pass by the sensor, giving the sensor's magnetic switch more time to respond. If the magnet is too far out, the computer may give erratic readings at higher speeds.
In practice: I have some computer sensors mounted near the hub and others mounted near the rim. Both set-ups seem to work just fine. The main factor for me as been the clearance between the magnet and the sensor.
My mountain bike has a wireless computer with the sensor mounted very near the hub. There is no issue with the distance from the sensor to the computer.
The sensors on most computers seem to be less fragile than the wires. Through the years I have had plenty of wires short out, but never had a sensor fail.
As a measure of my experience, I have 4 bikes with computers, and at least five magnets from old computers stuck to the refrigerator.