I agree with Orion. If you have a thermometer that you know is accurate try putting that in the water to see if you get a different reading. Do you have the stick on the outside type, or a real thermometer inside the tank? The sticky ones I don't like because they're always reading the outside temperature instead of the inside temperature and easily influenced by the temperature in the room or sunlight etc.
82 is not the end of the world for heat...but if your heater runs high like that you may want to turn it down. If its fluxuating a lot and just happens to be 82 right now and you know its not the thermometer going whacky, then I'd get that heater out of there asap before it fritzes on you and get a new one.
Make sure the thermometer is not right next to the heater...that can also cause your readings to be wierd...I usually put mine on the furthest side of the tank from the heater so that I can make sure there isn't much difference and one side isn't getting colder or something. (I have a bunch of 2 dollar kind that you stick on the inside of the tank, and then a couple of 5 dollar digital ones that are real nice too.)
If you have a heater that is submersible I believe it is best to put it in the tank horizontally (not touching anything) because it will heat the water more evenly. If you put it near the filter then, like Orion said, you'll probably have a more even water temperature. (but most tanks have plenty of water movement that it shouldn't make much difference unless you don't have enough heater for the tank)