Stability of USB +5V output

The USB specification states that the 5v is supposed to be +-5% under load, which translates to 4.75v to 5.25v. A simple power connection will provide up to 100 ma. (Up to 500 ma can be drawn, but you have to negotiate for it.).

Most IC's designed for 5v should accommodate the +-5% variation also.

The 5 V VBUS is definitely not stable, and will usually vary with the computer's load. In USB audio products, this creates all kinds of noise, presumably from audio frequency fluctuations caused by video cards, hard drives, etc. For these applications, a regulator is necessary. With your micro, it probably doesn't matter, but if you're doing any kind of analog to digital conversion, I'd also recommend a regulator.

The USB spec lists a worst-case drop of 4.375 V at the device, after a bus-powered hub, with transient drops to 4.07 V. Keep in mind that not every USB host follows the USB spec, so it could be even worse.

Wikipedia summarizes:

It is specified that devices' configuration and low-power functions must operate down to 4.40 V at the hub port by USB 2.0 and that devices' configuration, low-power, and high-power functions must operate down to 4.00 V at the device port by USB 3.0.

Provided you are using a computer as a source, then yes you can assume that the 5V is stable.

You cannot assume this if you use a mains power supply adaptor with a USB output, like the one that comes with an Apple iPhone.

Also, to use a regulator effectively, you should supply the regulator with a slightly higher voltage than the regulator output, otherwise as soon as you put a load on it there won't be enough 'spare voltage' for it to draw from to maintain +5V. (I know that's a bad way of saying it but it kinda makes it easier to understand!).