Just returned from a 2 hour presentation sponsored by my Company, listening to RockwellCollins WXR, B777, A320, A330, A340, all carry the same Multi scan units our no name airline uses. What an interesting lecture and presentation.
Morale of the story, Use Auto as it has so many inbuilt functions, including a memory in which you are in the world and then paints better pictures of the CB's you see. CB's differ around the world, lower over the oceans than over the continent, and so the RADAR in auto knows this.
He also said in auto it scanning at to different angles, one is down at minus 3 degrees and another higher. It puts cells into memory, so that even when you get closer and they would normally disappear, the memory will still scan it in auto and show you where the cells are nearby and below the aircraft.
He did say, use manual at TOP of descent, make a scan and look, then return to auto for the descent. (I can see some new SOP changes appearing on the horizon)