Your PC and monitor do not care. In all likelihood, they're specified to work on 100-240V, and it's unlikely your 120V is sagging all the way to 100V. 100V is the voltage used in Japan. And that 100V is just the nominal value: it's actually 100V ±10%, so as low as 90V. (This is why some power supplies even state 90V as their minimum.) If your 120V is sagging to 90V, you'd have to have massive problems in your electrical system.
Anyhow, due to how switching power supplies work, many of them work well below their rated voltage. (And on DC, too, since the first step is to rectify.) As an example, the Rigol DS1000Z series oscilloscopes are known to work on 48V DC, far below their rated AC input voltage.
If you get a UPS, get one if your power is unreliable and you have frequent blackouts or serious brownouts. Otherwise, don't bother.