I made a clean flash with the first release in october 2015, and never since.
Everything works well since that day.
My next clean flash will be for nougat when it will be ready, no rush.
You've got to admire a dirty flasher
Google's OTAs for devices that support it are the equivalent of a dirty flash and 7.0 is as painless as the regular monthly security updates, so you may want to dirty flash nougat when it's ready. I will only do a clean flash if any problems arise...