Personally, I like the look of the black background, it really makes the colors "pop."
On the other hand, there are quite a few articles out there about NOT putting ANY kind of background on the tank due to the reflective properties of things... Your lights actually get more penetration WITHOUT a background on the tank...
If I have some time later, I'll find a couple articles for ya... Research it some if you have time...
I am currently running no background. However... Even with the stuff said about the natural reflective properties of glass, I will most likely be painting my 90g before I set it up.... And BLACK.