Getting back to magnet placement...

Depending on how heavy the magnet is and how "in tune" you are with your bike, placing the magnet closer to the rim may cause an annoying off-balance shake, especially at higher speeds.

This is from experience: Planet Bike magnets are really heavy, and you can feel the vibration from having it mounted close to the rim when you get over 20mph. I keep mine as close to the hub as possible, typically mounted between the spoke crossover and the hub.