Took me awhile to relocate
this thread. Unfortunately, it is a victim of the great Photobucket marketing blunder and all the nice high magnification photos of the powder grains are gone, though the titles are not. The bottom line was he accrued 300 hours of tumbling of some rounds and they still worked normally and he could see no difference in the powders before and after. Again, a deteriorating powder may be another matter, but I don't have data for that.
Anyway, the warning never to tumble loaded rounds (if the powder is in good condition) has looked more and more like a wive's tale to me as time goes by. Still, I don't blame the powder companies for nay-saying the practice as there is no upside for them to saying you can do it. If they say it's OK, sooner or later some numbskull will tumble a case with a double-charge in it and then blame tumbling the powder for causing the problem, and by extention, the maker who OK'd doing it.