If you are buying pre-mixed e-liquids, they shouldn't require steeping. These are mixed in very large batches, bottled, and sit on a shelf in a warehouse until shipped to the consumer -- this sitting on a shelf is all the steeping that juice is going to need.
Some e-liquid manufacturers do not mix the juice until you place the order. These are considered "custom" mixed. You get to decide on the pg/vg ration, whether to add a flavor bump, add menthol, etc. These often have a "birth date" or mix date on the label. These e-liquids likely will benefit to some form of steeping.
Think of steeping like you would for wine fermenting, or when you make a pot of pot roast. That pot roast tastes pretty good on the day it is made, but will taste even better 2 - 4 days later after all of the ingredients have had time to add their flavor to the mix.
After over two years vaping and using a variety of many manufacturers' e-liquids, I find that I just allow new liquids steep in a cool dark place for 1 - 2 weeks. If the liquid has a perfumey or alcohol smell, I'll leave the top off for 24 hours to allow it to evaporate. Then I replace the cap and shake the bottle well, and let it sit.