5.1V seems to work fine, its mostly down to the usb cables quality.
I run mine from a separate switch mode unit @5.1 -5.2 and it doesn't flag up at all. Most of my projects are 12-14v battery powered.
Even running it from a 5v battery supply it still shows up as under volts!

hi just spotted something weird. With raspi-config if i go to raspi-config and then interface options ( no 3) on this one. if i select camera it does the camera function ok. if i select SPI it comes up Programmer error: unrecognized option if i select I2C i get the SPI page, and like wise for serial...