There are a lot of "flavorings" that have plenty of sweetness on their own and don't require any to be added.
I suggest making up 1-2ml of each of your flavorings by it's self and trying it.
I do that just to find how sweet, how strong, etc each one is.
You can do this without adding any nic.
I often have the same flavor from different company's, and testing them tells me who's is good and who's isn't
Even if I whip up a recipe, I won't add any sweetener until I try it.
It's easy to add if needed.
And YES, juice companies add sweeteners to pre-mixed liquids, depending on the juice.
Personally I believe the flavoring companies add it to the flavoring too, because a lot of them are sweet.
Necessary ??? depends on the juice. Sweetener will bring out some flavors.
It boils down to ..if you don't want to use a sweetener in your DIY, it's up to you ..you're the one
vaping it
What I don't like is a lot of them use Maltitol instead of Sucralose.
Maltitol is derived from sugar, and is just as bad as sugar (for me).
That's one reason I DIY, and try to stay away from the flavorings that taste to sweet on their own.