on on on on on on
Both Pg and too a lesser extent Vg are hygroscopic - which means they attract in water particles literally from the surrounding air - alongside anything that may be dissolved in the water.
The cleanrooms, Laminar flow units and air control in production environments are not just for show.
Ultimately .. you are mixing at home - its your choice .. but from a production standpoint seal it up as soon as poss - and let time be the greatest steeping agent.
Not sure what poll you're looking at but in the one you linked its 48-8 in favour of leaving the caps on.Seems a majority vote to leave the cap off here: http://thevapersshed.boards.net/thread/3703/steeping-poll-caps-on-off
Not saying the majority always makes the right decisions though. I have also read that plastic bottles, unlike glass, are permeable to air. This means that air particles can pass through the plastic bottles!