My friend that helped me get started in reloading said that tumbling was not important. He now shoots bench rest only and the brass never goes to the ground. When I started loading for semi-auto hand guns the brass always went to ground. At that time was loading for 357sig and the brass was not re-sizing very well, not long after that noticed that the brass were looking really scratched. Did some research and found out the brass was dirty and that is what was scratching the dies it came from the grit after being on the ground.
Yeah, I always say a good tumbling before starting to reload is needed (could be wrong, it's probably not needed either but I want my dies clean), but as far as tumbling after they are loaded, it's not needed, but it sure makes them look purty