There is nothing in the certification documents, that I am aware of, for aircraft certified for multi crew, that says the pilot in command must sit in the left hand seat.The design of some aeroplanes, the 737 for example, makes it necessary for the captain to sit in the left seat to access the tiller etc.
That said there is only one pilot in command - the person in the bunk is still the captain
So long answer to a short question, no requirement that I am aware of to sit in the left hand seat in the cruise as a relief pilot.