I've never steeped or stored juice in the dark. I do not store juice in sunlight. I tend not to have juice for more than 6 months, but I do have some nic base in the freezer in glass bottles which I think will survive to a ripe old age.
Here's an experiment done by Madvapes on
nic concentration over time with various storage options. The ones they say they kept on the 'Window Sill' were in sunlight during the day. From this data, IMO it doesn't matter what you do with juice for a coupe of weeks, it takes way longer than that to degrade nic.
I would imagine that for flavorings it all depends on the flavoring. I store mine in closed bottles out of direct sunlight (but not in the dark). I've had some for 6 months and had no issues with them, but that's not a scientific test. It doesn't worry me enough to do any more research into it.