A heater depends on where you live. If you know for certain that you can keep the water in your tank at a consistant 78 degrees, then no you don't need a heater. A fluxuating water temp can lead to illness, if you aren't sure, don't risk it.
A consistent water temp is a must no matter where you live. Either by using a water heater in the cooler temps or an air conditioned room or by keeping the water cool enough in the hotter climates. Fluxtuating water temps are one of the quickest ways to end up with a sick turtle. So as missibu said, depending on where you live, you may need a heater. If you do get a water heater, make sure it's fully submersible.
I hate to be the devils advocate here but if the temperature cannot fluxuate then how do turtles survive in the wild when the temperature goes up and down everyday?
When keeping turts indoors, we try to keep everything at the ideal temperatures. There is fluctuation outside of course, but it's not something we should risk with hatchlings. My turtles do just fine in 70degree water but I like to keep it at 75 just to be sure they won't be too cold. Better safe than sorry is all.
The things that come to those who wait may be the things that were left by those who got there first - Steven Tyler
Indoor habitats don't really mimic the natural environment now do they.. Unless you're willing to risk a sick turtle or tempt fate by trying to hibernate them, then you keep the temp from fluctuating as much as possible. I believe in the wild they'd bury themselves in mud and gunk found in the bottom of the body of water they reside in. So it's different in the "human" world of turtles
Like Spookster, I don't see anything wrong with a bit of fluctuation in the temps for healthy turts, as it would in nature. The air temps for my turts drop by about 5 degrees at night. It's prolonged temps that are too cool for the turtle that are more apt to cause problems.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-
Another thought to consider is that if the environment is not warm enough to your turts liking, he/she probably won't be inclined to eat. One of the most common inquiries on here is 'why won't me turtle eat'. Other than adjusting to the new environment, it is usually caused by someone who does not have their water at the correct temp. I understand a little fluxuation. Even with a heater my water fluxes between 76 and 80.