Is steam lost in boiling negligible? If I bring 20 ounces of water (2.5 cups) to boil in a kettle, and promptly remove it from the heat, is the amount of water lost (by volume/mass) a negligible quantity? 

To be more precise about my own intent for this knowledge; if a pour over guide for coffee brewing is indicating that there should be twenty ounces of water at 195-205'f, should I go through the steps of weighing out twenty ounces of was-just-boiling water, or will the displacement of water to steam be relatively non-impacting of the coffee extraction and final cup.

What are the various ways in which more steam will be displaced, and how can I conserve mass to minimize variability and improve consistency extraction to extraction?
This depends on the time you need to bring it to boil (water evaporates long before it starts boiling), the surface area of the water, and your definition of "negligible". Best test it with your own kettle. 

My kettle needed 2.5 minutes for boiling it, and lost 11 g out of 560, not quite half an ounce. It will probably lose some more while cooling. If you use an open kettle, a very wide kettle, or heat the kettle slowly, you will lose more.