Seems quite natural to me that anyone interested in flying, a pilot most of all, would like to have a say and perhaps try a hand. As GtE says, it is a matter of pre-flight briefing, as formal or informal as you like.
I remember, shortly after getting my license, I flew to the EBDT fly-in with my instructor who had now become a fellow-pilot. For the outbound flight I took the lefthand seat, as I had always done; and when we were to fly back, he suddenly came up with "my time to sit at the left" and I realised how stupid I had been, clinging to old habits without ever questioning them.
Often I find it easy to arrange matters. Myself more interested in navigation and communication than in actual piloting, I will happily let the other chap do the flying, and I've met several who were quite happy to leave the radio and the maps to me.