I don't think the temperature should swing more than three degrees or so, depending on how delicate your stock is.
I have to have lots of heaters, because I have a large remote sump area in my basement, and that area of the house gets chilly. Plus the water being pumped through hoses cools them off too, because I keep the house temperature in the mid 70's. Since I have a large water volume and it takes several heaters to heat the water, I am in a sense safer from heaters sticking on, because any one heater is not so powerful by itself that it could raise the temp dramaticly on it's own. When my heater stuck on I noticed that the tank was about 2 degrees higher than normal.
So, if possible, it is advisable to have two lower wattage heaters rather than one biggie one. If one malfunctions and sticks on, the functioning one will not turn on, and hopefully the malfunctioning one will raise the temperature slowly, as it would be underpowered for you tank by itself, giving you time to notice.