Pretty common where? In standard chemistry, liquids are measured by volume and solids are measured by weight. This is true even when making up standard solutions (probably the most precise measuring commonly done). Most of the solvent is added to the volumetric flask, the solute is added and dissolved, then more solvent is added to bring the total volume up to the calibration line.
With a graduated cylinder, there is little more clean-up than using your method, and a simple water wash/rinse is sufficient. The needle dripper bottles of flavoring makes volumetric measuring easy.
All the juice formulas I have see give percentages by volume. Unless you know the specific gravities of each flavoring solution, your "by weight" method is not accurate.
Like Dan, I will stick with ml's to the nearest 1/10 ml. If I wanted to be really anal about it, I have the equipment to measure in micro-liters - but it is not worth the effort. Even if only mixing 10 ml of juice, 0.1 ml is 1% of the total volume - and I can interpolate to half that or 0.5%.