I use a on-demand propane water heater and I have begun to use cold water for small tasks like rinsing my hands of flour or salt while cooking. If I am only rising my hands, not really washing them, then the water will never get from the heater to my hands before I am done so there it seems like a waste of the gas.

And when I am using a sink with a hot water tank the same thing is true. The hot water won't reach my hands before I am done, so I just reach for the cold.