Think about it: no matter how close to or far from the rim the magnet is, it goes around the same number of times at a given wheel speed. Therefore, it makes no difference to the accuracy of the computer.

What makes a difference is the closer to the rim, the greater the distance from the fork blade (in general)--which may affect whether you can get the magnet and its pickup in reasonable proximity. There's also a small increase in rotational inertia towards the rim, but at practical bike speeds, it should be negligible. If you're OCD about wheel-balance, you might choose a spoke "most opposite" the valve stem, but again, in practice the difference is likely to be marginal.