We can go a LONG way of talking about voltage drop (e.g. the long cable out from your PSU in your desktop, the various connector it get through to get to that usb port and the resistance in the usb cord itself and various other stuff)
and this is why things like the Apple 10W USB charger actually outputs 5.2V to compensate for the drop (when driving 2A loads)
And as much as it is complicated, we need to know that the mcu in the arduino will be quite comfortable as long as the voltage is, say, 2.7V-5.5V. (Arduino Leonardo). And add to it is that it consume little power, too, so you wont have to worry about voltage drops in most cases.
Whatever you are attaching, however, might be uncomfortable with the slightest fluctuations. In that case you should provide a dedicated power supply for it (pass the signal by connecting the ground and signal pin on both devices)
im not sure whether you can do a "data-only-cable" by not connecting the power pin on the USB. I might try that later. But if it work then you can have a good power supply for the entire setup and not worry about power flowing back into the usb port.