The short answer to your question is: If everything works as expected, then no action is needed.
"Dirty" flash is a forum slang. I'm sure at least 95% of stock ASUS firmware users "dirty" flash every update. The developer is the one to make sure things go as expected. If every update requires a reset of the device (router, phone, tablet, computer, etc.), then this developer has to find something else to do. In our case RMerlin knows things. He does recommend "clean" start because it's just impossible to know who did what. People here like to experiment and the results are hard to predict. Most of complaints are usually not related with RMerlin's work. And he doesn't have the resources of Google, Apple or Microsoft to address user related issues.