Robin -
Well, you expose my ignorance of how these things work again 
I understand that I could provide the ESP8266 a range of voltages (somewhere between 1.5 and 3.3v) and it should work OK, so a pair of AA's would suffice. But if I've got an ultrasonic sensor that requires 5v and I provide it with 4.5v, won't I run into the same problems I did when I provided it 3.3v?
Also, let's just say for the sake of argument I went with a 9v. My understanding is that I'd be in the same boat - I'd just need to provide two step-down converters/voltage regulators (one for the ESP8266 and one for the sensor) instead of a step-up and step-down converter.
It seems to me I'm going to have to step up or step down my voltages in my project no matter what, unless someone feels like making me cheap, easily available 3v ultrasonic sensors