I have a Radeon 7000.
So to be certain we should always use Cls (or another full screen redraw) after Flipping, as you can't be sure the Backbuffer still contains what it contained before OR that it contains what was previously on the Frontbuffer. (In most practical cases you would want to clear the buffer anyway to get proper animation, but I guess it is the theory here that counts.)
(Or instead of Cls you could of course make sure you redraw the entire buffer in another way, like in typical single-screen 2D game where you redraw a background image each frame.)
So to be certain we should always use Cls (or another full screen redraw) after Flipping, as you can't be sure the Backbuffer still contains what it contained before OR that it contains what was previously on the Frontbuffer. (In most practical cases you would want to clear the buffer anyway to get proper animation, but I guess it is the theory here that counts.)
(Or instead of Cls you could of course make sure you redraw the entire buffer in another way, like in typical single-screen 2D game where you redraw a background image each frame.)