I try to use backgrounds that complement the aquascaping (i.e. rocks and plants to match what is in the tank). I think it gives the illusion of being more natural and larger. Of course each background is also hiding all of the ugly filters, wiring, pumps, etc.
Gene Heitman - 12 tanks (11 freshwater and 1 saltwater), 206 gallons, 20+ species of fish/shrimp/snails, 52+ species of plants ... 10 years ago I just wanted 1 tank with some fish