My problem is, I have a sensor, which needs exactly 5.0 volts to function
If this is actually the case, then you probably don't want to rely on USB anyway. You should be looking at a solution that provides a more stable voltage reference (like a voltage regulator.)
Just because your USB port is 5.0v today, which is surprising, it will not always be exactly 5.0v. The USB Spec allows it to vary, at any time, between 4.75v and 5.25v.