All USB ports work on 5V. That said, some ports may not be able to deliver enough power to the PI. 

I wouldn't rely on it, but worst case scenario the port's polyfuse on your motherboard will open, and turn off the port for some amount of time. I doubt there would be any damages to the PC or the PI, but it depends if the motherboard has said fuses incorporated into the design (some older, archaic ones may not have them). You'll probably be fine (at least for some time). It's risky though.

A better solution would be to use powered USB hub, or USBv3 high power port if you have one available on your PC.

If you draw too much current from USB port the voltage goes down by a  significant amount, before the polyfuse kicks in. That's something to consider, since Raspberry PI's components doesn't work terribly well when on undervoltage. 

Another thing is, that the said undervoltage means heat - this leads to more power consumption. The PI can power off randomly under such scenario.