My evidence is purely anecdotal, but in our house I fill the tanks for both myself and my wife. I fill them completely full. My wife vapes them to the point of being completely empty with no noticeable ill effects. I tend to top mine off when they have about 1/8 inch of fluid left in the bottom. We both use the Single Coil (SC) 2.0 ohm cartos most of the time, but for some flavors I prefer the Dual Coil (DC) 1.5 ohm cartos which come with the tanks originally.
Hope this helps!