My preference, whether using wide back or pieced backing, is for it to coordinate in some way with the top. I use "coordinate" quite loosely - it may be from the same line, but more often is not. If the top is predominantly blue, I'll use a blue print.
When I find great deals on fabric, usually clearance, I'll buy several yards with the intention for them to be used as backing. Sometimes I've done large pieces of leftover fabric, but not usually. Don't know why, I don't care for muslin or solid; I like the way a print looks. Living where I do, I am fortunate to be able to buy wide backs for the price that many folks have to pay for regular yardage.