It really varies from one e-juice manufacturer to another. The pre-made juices (most probably made in China) will need no steeping. They likely were made in large batches, cut into smaller quantities for small delivery bottles, and made their way to their eventual destination. That in itself took some time.
The ones that I have found to benefit from steeping are the juices that are customized to your specifications: specific nicotine strength, pg/vg ratio, extra flavor, added sweetner, added menthol, or anything else that you might request from the mixer. The organic juices also seem to require some steep time. Many vendors will include a "birth date" when the juice was made -- that's a clue that the juice will require some steeping.
Nothing will be hurt should you wish to try the juice upon delivery. Just be mindful, that should it taste unpleasant, or not what you expected, do not discard the juice as a bad juice. Allow it time, 3-4 days, one to two weeks, depending upon the juice, to steep. Then give it another shot. It very well may surprise you the change that took place.
Everyone probably has a different method or ritual to steep their juices. I usually will test a small quantity in a ce3. Then I will put the rest away with the top open to air overnight. If the juice was made with an alcohol base, this will get rid of any perfumey or chemical smell/taste. I will re-cap it the next day and shake it well and put it away out of sunlight and test it again at a later date.