aerobob-- There was a thread here on RCU a long time ago that debated this. Some aerospace guy said that the gas in the tank "tends" to stay in the back on a down line and he had some video to show it. As you mentioned a down line at low throttle setting should not empty the line even if all the fuel went forward.
The example I mentioned happened more than once to me. The knock off tygon went stiff, as in solid, in about 3 months on one of my planes. When the tank was less than half full and you did extended inverted flat spins the engine would die. When I finally decided to look at the tank, the clunk would not move from the back bottom. It would take a while before it died..