My tanks are against a wall but I use the black background anyway as I have HOB and wires to shield. I also did not want the color of the walls to interfere with the tank. I did not really think of light loss when I did it.
If you want to view the tank from both sides, which is a cool look, go for it. If you have the correct lights anyway your tankcoral should be fine no matter what. Some folks I have read use spot lighting anyway to directly get the correct light on the target specimin and do not flood the entire tank with light.