I'm not aware of the details about how the ID is used. If its something the user will see at some point, then it should be the same to avoid confusion. 

Round robin means that if you have a list of services, then you will use them in circular order without repeating them until all the services had been used. If the state information is stored in each node, then that won't apply.
On the other side, maintaining that info in sync all along the cluster will impact the performance, so maybe the user would like to choose whether he wants cluster/node round robin scheduling.