I would recommend supplying power to the Tx path even though it not going to be "used". You may find that your use scenario will require internal Tx blocks.
Tx outputs if unused need to be terminated. The recommended approach is to pull them up to 1.3V supply voltage.
UG-671 document provides detail related to enabling different channels and blocks within the chip. For example function ad9361_en_dis_tx that controls enabling/disabling Tx channels. You may find it useful for optimizing power consumption.