My source voltage is 5v-12v. My current idea is to have a switching regulator (TPS54302) that provides a 4.2v supply, following which I use a LDO to provide a 3.3v supply to the ESP 12.
One of the reasons for doing this, rather than a direct 3.3v is that I would like to power 2 WS2812b LEDs as well, so I could use the 4.2v rail for it.