My view is that it depends on the pass through and the power source. USB power does operate at 5v, period. The trouble is most PC's and adapters are not able to deliver sufficient current (amps). The interface was originally designed to provide just 500mA (in reality, they may provide more, but perhaps not 4x as much..) and therefore the voltage under load dips, as any atomizer at 5v will require more current than this.
A decent 2 Amp + power supply should operate at very close to 5v under load (2A @ 5v is 10 watts which is at the upper end of the so called "sweet spot" for atomizer power). Also the very cheap Chinese pass throughs sometimes use very flimsy thin wiring that I suspect would impact on the performance. Most of the pass throughs featured on
this page (except the Tornado) should operate at 5v when fed with a sufficient power source.
2A USB power adapters are becoming common because they are needed to support high speed charging on things like the iPad.