The key idea is to install both somewhere accessible and convenient to you, and representative of the temperature in the room. Mitsubishi provides some additional guidance on this:
From the remote installation manual (applies to remote sensors as well):
� To monitor the accurate indoor temperature, install the remote controller away from direct sunlight, heat sources, and the supply air outlet of the air conditioner.
� Install the remote controller in a location that allows the sensor to measure the representative room temperature.
� Install the remote controller where no wires are routed around the temperature sensor on the controller. (If wires are routed, the sensor cannot measure accurate indoor temperature.)