The need for flushing might depend on how your tank is plumbed. The drain for mine exits at the forward side of the low end, not directly on the end. Stuff tends to trap along the low end of the tank. From there it makes a 90° bend where it joins the gray tank drain, another 90° bend to join the galley tank drain and another 90° bend to exit. I'm not as concerned about all the 90° bends as I am the fact that it exits out the side of the tank.
I have learned how to back flush using the gray tank and the right combination of the three knife valve's positions and save using the flush system for when I'm back home. Amazing the chunks of this and that come floating by the clear elbow.:E