IC voltages and low power design, What todo?
Ideally, it would be best to keep the majority of your circuit at the same voltage. The reason alot of datasheets will have the readings at 3.3V (for a 1.6 - 3.6V device) is because that is a commonly used voltage. You will find a lot of datasheets will have multiple graphs showing the operations of devices at different voltages and currents. Mostly you will find the voltages to be 3.3V, 5V, 9V, 12V and so on, as they are very common. I would just stick with the 3.3 if I were you, as you can get a known performance, and some devices may not behave as well in the lower supply regions (depending on what load you are driving etc) And if yu want different supply voltages to different ICs, you will probably need to level shift signals, so I wouldn't bother.

Stick with 3.3V, or if you are really concerned, you'll just have to perform your own tests at different supply voltages.