Chamfering enough to eliminate an overly sharp edge, or burrs is OK. Colt for a time chamfered the chambers, but then stopped. What reason the start or stop of the practice, I don't know. Shaving a ring or swaging with a chamfer wouldn't noticably change the flat put on the ball, as the difference in diameter between the ball and the chamber is what determines the flat, and you need the difference to seal the chamber..
In any case, if you feel you must, only cut a very small chamfer, no more than pascalp showed, as you don't want it to contribute to the fouling driven down to the cylinder arbor at the cylinder gap. I chamfered a few, but generally don't, as I've had no problems with factory chamber mouths.
The exception was Ruger Old Army that swaged a ball smaller at the mouth than at the seated position, and the ball would fall out of the cylinder once seated. I sent it back to Ruger and it was rather crudely scraped out with a scraper, something I could have done without the wait involved with shipping. I had expected a new cylinder........
