It’s an interesting thought for sure, but I agree, 12V or 5V it really doesn’t matter. When thinking about battery life what matters is total current draw. Which you can determined from the wattage/power consumption of each device. Ohm’s Law can help to work that one out.
Here’s a good write up on Voltage.
I’m not sure, but you might want to also take into consideration the power consumption by the step-down. Perhaps it is a negligible value but when dealing with low current devices such as LED’s I’m curious if there would be any net savings.