Steeping for me is done in a box that blocks light and I leave the cap on the bottle. Steeping just allows the ingredients to thoroughly mix over time. Some vendors mix up
juice in advance so that they have inventory in stock. Others mix and bottle it at time of order processing in the name of efficiency. No one wants to be caught with 100 bottles of
juice "x" that generates no orders while juice "y" is in demand and some of the ingredients were used up to make 100 bottles of juice "x".
I think of steeping like I do spaghetti sauce. It's always better the next day or the day after. In the case of ecig juice, it's better two weeks after. If a vendor makes it a week or two before you order it, then it might just be great right out of the bottle when you get it.
Shaking is a good habit. Shake it well before you open it and a few times each time you refill a carto. When they are really full and won't shake, I roll it and invert it and let the bubble do the mixing as it moves in the bottle.