Yes, I think that the notion is that this insures that the flat bowl will be full, and so the float needle valve closed, so that you don't get "reverse flow" whereby propane backs up into the gasoline tank, creating an fire / explosion hazard. Back when I carried my Yamaha 2400 tri fuel around with me, I just always carried it with the gasoline tank FULL - on the theories that a.) this would allow for less gasoline oxidation / degradation, and b.) if ever I ran out of propane, I could still run the genny on gasoline.
At some point, I discovered a third reason: my machine is sometimes hard to start on propane, but if I run it for a few minutes on gasoline to warm it up, I can fire it right up on propane ... and then fiddle with the mixture for a bit to get it right ... (silly me, I thought that once I got the mixture leaned out properly and the mixture knob "locked" that it would always be fine in the future at that setting. Not so, at least on my machine.)