"Steeping" - misnomer though it is - does not guarantee an improvement in quality; in fact, it doesn't guarantee any meaningful change at all. Of course, that isn't meant to forbid the possibility that some change can be observed in some cases, you follow? It's been conjectured that volatile flavor compounds may evaporate more rapidly than others, causing some aspects of a flavor profile to diminish. There is also potential that some compounds cluster together in freshly-made juices, and require agitation (heat or shaking) to diffuse them throughout the mixture. If the "cluster" theory is really what's at play, it's possible that duration of "steeping" is less important than agitation of those compounds. Diverse e-liquids potentially comprise multitudinous flavor compound combinations, so an observed change in one variety does not necessarily predict the same effect in another. Absent quantifiable data, the best guesses ventured seem to suggest that flavor change (not necessarily "improvement") is observable in only some types of juices, and is more likely at younger ages when diffusion of flavors throughout a mixture has not run its course.
The best thing you can do for yourself is to maintain an ordered and skeptical mind. Keep records of the methods used, time elapsed, and type of juice, and do what seems most subjectively satisfying in your own experience. The other option is to buy only juices that taste good right out of the box, and spare yourself the trouble. Either way, have fun.