If I remember right there's a bit of wiggle-room in what devices consider a logic high/low. I don't think it HAS to be bang-on 5.0V, just above a certain threshold. I power my stuff using LDO 5V regulators and usually, unless it wants a lot of current, a PP3 battery (because I have hundreds of the bloody things), and it always ends up pretty close to the 5V mark 