It really depends on the fish, some are more sensitive than others. But for average community fish, I wouldn't be concerned unless it starts fluctuating 4 degrees or more. There is usually a small fluctuation at night when the lights are off (unless you have flourescents), and smaller tanks are more likely to fluctate than large tanks.
I was also wondering about this, because my tank was always 24-25 degrees, but with the cold weather, the temp drops at night so I upped my temp to 27-28, but some of my fish now doesn't look very well!!! I don't know if I must change it back and if this will be another shock?