back in 2005 we had the great north american power outage and I had no power for 13 hours.
had heavily stocked tanks with large barbs, silver dollars etc.
had no issues, not even a post ammonia spike or anything. however, I had just done water changes the day before.
the large barbs were, however, breathing pretty heavily (almost gasping) by the end.
I went out the next day and bought a few battery powered airpumps.
I have yet to have to use them, but they are there in case.
at the time however, I worked at the LFS who didn't have power for 22 hours. mine came back early.
we lost one of our big sharks (a 5 foot lemon shark) because the generator we had was out for repair. figures the power outage would happen then.....
but we had ammonia spikes in many of our 'pods' of tanks.
so, it can definatley be an issue.