I've also tumbled many thousands of rounds of both pistol and rifle ammo and never had my reloading room explode. And yes the major manufacturers DO tumble their loaded ammo just prior to packaging. I've also seen bits of cob stuck between the primer and case in several different brands of ammo.
Here's a link to what I consider an exhaustive test of this issue on AR15.com a while ago. Read it all the way through, and judge for yourself if tumbling changes any of the burning characteristics of powder.