For variable voltage devices, namely SD cards, how much power do you save by using a lower voltage?
Circuits that have a switch mode power supply (either a DC/DC converter, or a switched capacitor regulator) will not change the total power consumption significantly as the input voltage changes.

Circuits that don't include a pre regulator will generally consume less power when powered by a lower voltage. When you reduce the voltage, most digital circuits will reduce their current proportionately, so the overall power will decrease as V^2 (lower V and lower current will both reduce power). Analog circuits are generally biased differently and while the power will reduce, the current may remain the same. That is still lower power.