For pistol, I definately tumble before decapping the brass.
For rifle, I go in 2 steps, I lightly rumble first to get rid of dirt, carbon deposits and grit, to avoid scratching my sizing die. Then I decap, lube, resize, clean flashholes and trim. Then I retumble to get rid of the grit generated by cleaning flashholes and trimming, and to get rid of the lube still on the brass. Then I run all pieces through the universal decapper to get the corncob out of the flashholes, then proceed to loading.
Long process, but it makes good quality, accurate and good looking ammunition. (I like my rounds to look better and shinier than factory ammo)