Is the thermometer on the outside of the tank? If so, the water inside the tank could be considerably different than the actual water temp. I did a comparison by putting a thermometer on the outside of my tank and one on the inside. The one on the outside reads a good 4-6 degree difference than what is on the inside. In the summer, it reads higher than the inside, and in the winter, it reads lower than the inside. Yesterday the outside one read 70, while the inside one read 76 (We live in an area that gets well below 0 in the winter and heating our house is very difficult - without a heater, my fish would have frozen long ago) I have yet to see them say the same thing.
So, it's much better to measure the actual temperature of the water inside the tank, rather than one of those stick on ones for the outside. But if you find that with the inside tank thermometer, temperatures are always consistent without a heater, I would say you don't need one.