The RTS is recommended whenever the temperature at the battery will be more than 5°C (9°F) different than the temperature at the controller. If an RTS in not used, and the charge controller does not have an ambient temperature sensor (Tristar, TristarMPPT, Tristar MPPT 600V) then the controller will not compensate charging setpoints for temperature.  If your controller has an ambient temperature sensor, then the controller will compensate charging setpoints for temperature, although it is best to use the RTS and measure the temperature of the battery.