It depends on the yeast and on your preference. If you don't want a lot of esters, ferment on the lower range. If you prefer more of them then ferment on the higher end.
For example the White Labs American Hefeweizen (WPL320) has a range of 65 to 69 degrees F. If you ferment in the 64 to 65 degree range, you will minimize ester production and the Hefe will be missing some of the flavors you would normally expect to find in this style. If you ferment at 69 degrees it will have a different flavor profile. Which one is correct? It depends on what you want.
Fermentation temperature consistency is important, maintaining a consistent temperature will ensure your yeast perform as expected, and generate the desired flavor profile.
Cheers!