Thats a good question, and is discussed in many threads on a number of forums. In short, steeping is an actual process. Air (specifically O2) isn't necessary. Esp hand mixed custom juices need time for all the parts to settle and mix well. You will hear all the time how one juice will taste off out of the box, but let it sit (up to a month for some of the more complex mixex) and they are great.
When I mixed, I did a couple of things to help the steaping along. I'd put the bottles in my ultrasonic cleaner for a couple of hours. The heat and vibration helped accelerate the process. You can do similar by running warm water over the bottle and shaking.
Now, as far as air, the oxygen will cause the nic to oxidize and eventually taste bad. For full bottles, this will take a while, and at room temps, this will take maybe, 6 months or more.
There is more than one oppinion on this and I'm sure others will chime in. just my take and observations
Oh, and welcome!
Sent from my GalaxyS 4G with my non GRIP'n hand.