Do you mean affinity?
It is not mandatory, nor is load balancing (the system is supposed to handle one server down, ie forcing 100:0 balance). But running permanently with 90:10 is probably not ideal as many clients will get less than performance than if load balancing was something closer to 50:50.
But with redundant servers it is recommended to deploy affinity since it results in a predictable failover order and can be used to ensure that a group of adjacent nodes don't choose same server by accident (running without affinity results in random connections).
Search AKS; there are examples of using node groups which makes it easy to create a well defined scheme, eg odd and even servers/clients.
Important: terminate each affinity row with "All nodes" to prevent an affinity that may lock you out from a server by accident.