If you have a 3port directional coupler and you are not using the coupling port, it's better to terminate it with a 50 ohm load for various reasons:
1. Depending on the coupling value, the higher this is (4dB for example), the coupled signal will reflect and couple back to the main line worsening the performance in terms of IL, VSWR, etc.
2. most of the coupled signal will reflect if the output port is not terminated and then this will couple back to the main line but most (depending on the coupling value) will go to the internal load. You may need to derate the input power in order not to damage the internal load.