I used to steep flavors open but then I started using DW, so I don't. With that said, I don't mix tobaccos either, but yes, juice will darken and change over time as it is exposed to air. I usually find if I let a full tank sit for too long I'll need to change the eliquid and clean it, as perhaps flavor rises initially but over time it will drop off more and more.... To the point that if I'm really not gonna use a tank for a while and I know it, I empty it. Easier to start off clean and then refill rather than the reverse.
I don't know the whys/wherefores and whatnots, either. I believe that most things have a cause, however, even if I don't know it, or even if anyone doesn't know it... Yet.
When I was air steeping, it was usually only for a day or so, for the most part now though, I steep with the lid closed and keep my (dark amber) e-liquid bottles tightly closed. I really need to get some more of them...
Anna