In article ,
says...

I have recently purchased a new CatEye computer for my bike.
In the past the computer that I have installed stated in the
instructions that the magnet should be placed as close to the
axle as possible. With my new computer the sensor/magnet is
placed about midway on the fork. Nothing in the instructions
state how high or low on the fork the sensor should go. If
the sensor is placed at a random height, won't this throw
off the accuracy of the computer? If the magnet is not
placed as close to the axle as possible, won't this throw
off the balance of the wheel?

One wheel revolution is still one wheel revolution no matter where the
sensor and magnet are. However, the placement of the sensor/magnet has
nothing to do with accuracy, but with how fast the magnet passes the
sensor. If the magnet passes the sensor too quickly, it may not have
time to register. That's why most instructions I have seen recommend
placing the sensor lower on the fork and closer to the hub.

I have used Cateye and Planet Bike computers and both of them work over
a fairly wide range of positions. The Cateye magnet is very small and
light. I even bought spares for my Planet Bike computers for that
reason. Bike wheels don't spin fast enough for wheel balance to be much
of a problem.