When we went on one, the guide announced that it was by rotation, ie everyone moved back two seats every day.
On our coach the people who'd been at the back on the first day, got to the coach early and sat in their new seats at the front. Everyone else then complied and moved two seats back every day.
On the other coach, the people who'd been at the front on the first day got there early and sat in the same seats. Everyone else was very British and refused to complain in public (although they all moaned bitterly in private) so they all ended up sitting in the same seats every day. The guide did nothing to enforce the issue.