Steeping - it is believed by many that this process helps many blends mature, or settle. It is the process of letting a freshly mixed bottle sit, preferably in a cool, dark area, void of sunlight. Some people allow 24 hours, some wait a week, (and some even wait over a month), depending on the blend, and flavorings used.
Generally, most tend to steep tobacco flavors, though some will not try their freshly mixed
juice for days-weeks at a time. If you're using many flavorings in one blend, it is generally believed that steeping allows the flavors to blend further, as opposed to
vaping it straight off of the mix.
Breathing, or airing - is the process of leaving the cap off of a freshly mixed bottle (generally overnight, for one evening). This is usually done with flavorings that are alcohol-based, and helps evaporate some of the alcohol (which can give a harsh throat hit, and sometimes a altered taste to the blend).
Now, let me tell you what I think of both practices. Bear in mind, it is my opinion only, and doesn't reflect the majority of the community.
Steeping - Unnecessary
most of the time. I
always taste my freshly mixed blends, and it's so rare that I steep before I vape my blends. I hold firm belief that they should taste good immediately, and if they don't, there is little chance that it will improve by steeping.
Airing - I refuse to do this because it is also generally known that while you're evaporating the unwanted elements in your blend, the flavoring itself is also evaporating with such a process. Of course, the alcohol will evaporate long before the flavoring, but it is not a practice that I feel is needed, and I use such little flavoring in my blends anyways, so I do not wish to hinder them any further by leaving the cap off.
My 2 cents, nothing more.