<p>Specifically using stainless steel nozzles, but I guess it's worth knowing about brass too. Is there any reason to be concerned about dimensional accuracy of the nozzle or anything like that as a result of repeated heating with a butane torch? </p>

If you carbonize the filament or other particles that are clogging the nozzle, then you will never get them clean. In my experience, it's not worth cleaning the nozzle with anything other than cleaning filament. If that doesn't work then change the nozzle. Heating the metal nozzle with a torch will change the temper of the material. I used to try cleaning with a soldering iron; which was to no avail.
Purchase a dozen brass nozzles and save your self the headache. Either that or a good quality set of stainless steel ones. They are easier to clean with the cleaning filament and aren't ablated by the filament as quickly, allowing for better dimensional accuracy over multiple prints.
https://www.amazon.com/eSUN-CLEANING-Filament-Printers-Cleaning/dp/B00MVIYNFW/