What happens if a node joins a cluster before it is ready? Fortunately, you'll never have to find out. For this very reason, there is a test suite maintained by the Kubernetes community called the Node Conformance Test. This set of tests will ensure that the node complies with at least the minimum requirements to join a cluster.
Of course, this only applies to you if you are setting up unmanaged nodes (i.e. nodes you have to create/setup yourself). Otherwise, if you're using a managed cluster and nodes, you'll never have to think about this.