Generally speaking you usually will get away with a power outage. It all depends.
The power coming back is where the damage usually occurs as that is when uncontrolled surges are possible. With the ATX power supplies, they usually will take the hit and die leaving your computer ok. Not always, but most of the time.
As cheap as surge protectors are these days, it never hurts to have one. Your television and stereo equipment are more likely to be damaged than the computer.
Just remember to replace surge protectors once a year or so. They take less surge as they age.
