I have gotten custom juice that benefitted from steeping. Most storebought juice often doesn't require it, but if you are detecting harshness or feel as if the flavor isn't really melded, well, you can certainly not vape it for a week or so. Some of my storeboughts improved with age, and some of that may have been due to offgassing (IDK) as the level of the juice dropped and more air (probably) would enter the bottle.
With all that said, steeping may or may not improve a juice and it can be a bit individual. When I DIY I steep, and fairly thoroughly (I try to time batches so that they steep about a month). If you are an "impatient' vaper, well yes, an UC can speed steep time (sometimes at the cost of other elements, however, depending on how long and how hot your settings are). You really want to not apply too much heat to mixes.
I will agree with whomever stated if they had a juice they really loathed from the first taste, it can be somewhat unusual for steeping alone to remedy the issue, but it can certainly "improve" some juices. That's highly individual, so I would just check things out, keep tasting, and keep notes. If for example you find a storebought juice "perfect" after 2 weeks and 2 days, well that gives you a guide for next time (with the caveat that, depending on where a juice maker is in their "vat" so to speak and whatever) your juice may receive different steeping times depending on where you are with the "batch" of juice being made and etc.
I do think some juicemakers plan ahead and steep as they need to depending on order status, but probably not all of them....
Best of luck,
Anna