AFAIK there is no legal requirement whatsoever that the PIC sits in the left or right seat. Nor, if somebody is under instruction, to sit left or right.
Mind you, there might (note: might) be limitations in the POH that if an aircraft is flown solo, it has to be from a certain seat due to W&B considerations (mostly applicable to tandem seating and light helicopters).
In some handbooks there is a requirement that is should be flown from the left seat, as some of the controls are operated from there.
I couldn't fly a PA 28 solo from the right hand seat as the fuel cock would be out of my reach.