Backing on the cheap
This is veering off the original course, but one factor in choosing backing can be economy. If you have a lot of reels, the total cost of filling them all with premium dacron or gell-spun backing gets painful. I've bought and equipped a number of second-, third-, and forth-string reels and lines in the last few years, mainly with the money-saving help of eBay. In a lot of cases, a used reel has come with the bonus of free backing, and sometimes with fly line and all. Free is a great price.
(I've found strange things on some reels: a fly line over cotton twine backing - over another sinking fly line ...but that's another story.)
Part of the lore of tackle rigging is that monofilament should never be used as backing, because it stretches; and if wound on under a fish's tension, could warp and crush a spool hub. Maybe, but I've never heard of that happening; have you? (One of my purchases had mono of approx. 60 lb. test; I doubt if that could have been stretched dangerously.) For some of my light line trout reels, where the chance of fighting a big, strong fish are remote, I used 25 lb. test nylon kite string. I bought a 200-yard spool at a grocery store for about $1.98.