It isn't best to steep in the dark. Though it depends what you want.
Some things like bread, are best made slowly. When you make bread you make the dough into an elastic very stetchy paste and leave it for the yeast to do it's thing. The cooler the dough is the longer it takes for the yeast to multiply, but because of the way yeast works longer is better as more flavour will develop in bread that is proved at a lower temperature vs bread that's proved in 'ideal conditions'
When you steep juice what's happening is a chemical reaction between the nicotine and the flavour compounds that are present in the e-liquid. This reaction Smooths out the rough edges of the flavour and lets things mix together properly. There are things that will make this reaction happen faster (see the
UV Steeping thread) things like UV light (which is in sunlight) and warmer temperatures (sunlight will warm the e-liquid slightly) and increased air (leaving the bottle open or blitzing your e-liquid with a milk frother)
While there may be a difference in quality in steeped over a long period of time and steeped using some kind of accelerated method, it's going to be a very slight difference.
Once they're steeped
then you ought to keep them tightly closed in a cool dark place (locked out of the reach of children ideally) but if you're steeping in sunlight it'll take several months before the nicotine content degrades enough to notice and typically you are going to be steeping on a windowsill for example for a week at most.