I think it depends on the tank and where abouts it is. I have a planted tank that I painted solid blue. I also have another one that has a styrofoam background ( home made ) and my 6 footer is clear. I think they all look realy good.
i prefer a deep blue for a reef tank, but for a planted, i like a nice planted background, it adds depth and texture, makes your plants blend in with the back of the tank, not stick out like a sore nut.