The venturi should be set at minumum when at the surface, when used as an octo, or out of your mouth for any reason. Where it is set during a dive is up to you.
Basically, it does nothing when air is not flowing in the reg. When air starts to flow, if it is set off the minimum position it directs air in such a way as to produce a slight vaccum inside the second stage, the higher it's set the more vaccum it produces. This vaccum assist the diver making the work of breathing less. The down side is if it is set to max and the air has no back pressure (like it does when it is in your mouth) the vaccum on most regs is enough to cause the reg to go into violent sustained freeflow once the air flow has been started by some external force like bumping the purge button or water hitting the reg during an entry. Hence the reason for always keeping it in the mini position when not in your mouth. When set to max, a reg should violently free flow if the purge button in pressed and released while on the surface and should continue to do so until something is done to stop it...like creating back pressure by placing your hand over the mouthpiece.
I personally keep mine set to mini when out of my mouth and at max when it's in. Depending on the reg and your preferences some feel it will "force feed" them air if set too high, this is a decision only you can make.
