Originally Posted by GaryBy

The sensors don't measure speed directly. They just count the number of times the magnet goes past. For cadence, that's all you need. For speed, that's the reason you need to program in the tire size.

The main requirement is that you can get the right gap between the sensor and magnet, without anything getting in the way of anything else. Usually this means going part way down the fork in order to get them close. For the cadence, it may depend on the shape of your chain stays, but typically you need to go towards the back. The secondary requirement, with a wireless, is getting closer to the computer. So go as far as necessary for the gap, but no further.