Most just paint the backglass, and/or let the inevitable coralline algae cover the backglass. To me this looks hideous, and some effort to keep it clean oughta be made, even thought it adds work to a hobby that is supposed to be 'fun.'
Aside from the fact that it looks hideous, there is also another compelling reason to keep the back glass clean.
A while back, (read years) an ariticle was published in TFH magazine that dealt with the question of painting the back of tanks, etc: "Reef Aquarium Lighting: A Look At Backgrounds: To Paint Or Not To Paint." The article is not in the online archives, and, even a search by title, does not bring up the article, only references to it in numerous forums. Direct links to the article are dead. The underlying theme of the article was that light is not always doing what you preceive it is doing. The assertion of the article, was by painting the back of the tank, the reflectivity of the glass is reduced, thus reducing the amount of light in the tank. The physics tend to support the assertion, so culturing algae on the glass would further reduce the reflectivity...
Reef 'depicting' backgrounds bring back fond memories of sunken ships, plastic plants, bubbling treasure chests: all looking as fake as they are.
For the industrious, that don't mind a bit of work to keep their tanks first rate, and provide a better 'illusion' of the natural reef, perhaps this will suit their needs:
http://reefcentral.com/forums/showthread.php?t=1868825&highlight=background