I used a slightly rounded/angled crimp on my patched ball loads, in annealed brass cases.
I also wondered if the patches would stay with the balls as the ball left the base mouth.
This appeared to be the case, as evidenced by the after shot patch retrieval, just as if they'd been shot from a muzzle loaded gun.
There is some speculation that the crimps open up as the case expands with initial ignition of the powder, even before the ball or bullet
starts to move. Of that I have my doubts, but, they seemed to work just the same. The patches I used, came right to the top of the
case mouth, well around the periphery of the loaded ball. This itself, might have been important.