weight is always more accurate, when I make cosmetics I weigh all ingredients as density of different liquids is way off. Castor Oil for example is gooey sticky stuff, while Almond Oil is like water. And if it is cold at home, the ml amount makes quite a bit of difference to when it is warm in my house. Also, weighing out something like a teaspoon is highly inaccurate, as sometimes they are filled better, sometimes less.
So yes, weighing would be the best way to do things. But 1ml is not 1g, that is only true for water. Oils and alcohols (PG is an alcohol) have vastly different specific gravities and will weigh more or less than 1g at 1ml. Oil will be less (apart from Castor oil which is more) than 1g, alcohols are even more less than 1g and flavors would most likely weigh more than 1g (I would assume so, as they are water with bits added).
So definitely, if you want to have repeatable results, go and weigh your ingredients. But one tip, it's easier to make a larger batch, than a tiny one.... hence me making body lotions at batches of 1 litre only... just cannot be asked to weigh out micro grams of something (when you buy body lotion in the shop, 70% is water, 25% is oil, and the remaining 5% is all the active and fancy ingredients that are advertised on the front of the bottle, and this is for the expensive stuff...cheaper body lotions have all the fancy ingredients in the last 1%!!!)
