That's always a tough choice.
If your equipment is suitable and you've got a light-colored plain wall behind your tank, then you can easily make due without a background. Or, you could add some of the light dispersing film, that helps to make for the "blurred distance" look.
However, if you're using things like HOB filters with dangling cords and things behind the tank, then it often looks better to apply a background so you don't see the filter/light/heater/o2/etc cords. Helps to keep your eye from being distracted by what's behind the tank.
I personally prefer to not use backgrounds, as I appreciate the extra depth perception you get from seeing the wall far behind the tank. Also, if you're not using a background, or just using a light dispersing film, you can lay a small T8 bulb/strip of LEDs behind the tank and let it shine upward, illuminating the wall... Makes for those incredible glowing horizon shots.