I add the nic each time. But I use 50/50 nic/vg mixture to get it to 50 mg/ml. I don’t like handling 100 mg/ml. If the flavoring percentage changes and you use a premade base, wouldn’t that change the final nic content? Unless you used the same % of flavorings each time. I’m also a non steeper of everything. But lately I have been warming my juice in warm water after mixing to do a mini steep.
Another good reason to premix an unflavored nic base, not having to handle the 100 as often. And not having to open it to air as much, either.
Yes, when you later add flavors, it will drop the nic a tad, and increase the PG (probably) a tad. But unless you're running around 20% or more flavor, won't make much difference. At my nic level, it's not really a concern. For those who vape at low levels of nic, it might be noticeable. Playing with one of the mixing calculators would let you fine tune.