Buying that way (and you certainly can) may be more convenient. As long as you can handle the slight changes when you add flavorings, you can certainly mix in that fashion.
I choose not to as a) it is also the most expensive way to mix and b) given what the FDA is looking at, as far as space, I can store (in the freezer, to preserve the nicotine) at a higher strength and then dilute it down as needed. The FDA is proving itself a bit unpredictable these days, so my preference is to use 100 mg/ml nic and to have a fair amount of it stashed away.
Of course, this doesn't apply if you aren't in the US, and you may decide that the convenience outweighs any concerns you have about the future. Every vaper has to make their own decision on that, and perhaps a happy medium might be having a few liters of higher strength nic frozen in case the worst occurs, and continuing to mix the way you wish unless/until you can't and etc.
Just my thoughts, and while buying nic that way overall is slightly more expensive the convenience may outweigh any cost, and it should still be less expensive than buying storebought juice and etc.
Just my thoughts, as always,
Anna
