Windows patching on 3 node Always On Availability Group
To be honest there is no definite answer to your question.

If you do the last fallback to N1 you tested it on all available nodes. Within limits, more testing seems to be better to me. But to be honest it might not be absolutely necessary.

Leaving it on N2 will get you the added benefit of using the N2 node for primary workload until the next update.

It might even be feasible to do the fallback to N1 and then do another failover to N2 to test if N1 is running and then have N2 as primary until the next update.

In conclusion: I don't think it is absolutely necessary to the fall back to N1 but if you can afford the time during your maintenance window I would consider just doing it (and maybe even do the failover to N2 afterwards).