In a send port the map is executed before the pipeline. This is the opposite of the receive port where mapping is the last step after the pipeline.
There is no best approach for choosing to map in the orchestration or send port. It mostly depends on your application. If the output of the map is some canonical message that may be used elsewhere you may like to map in the orchestration. If you are using a role link or multiple send ports, one to each business partner, then you may have a partner specific map in the send port.