Like many people have eluded to in previous posts I think it depends on a combination of things including: how you treat/handle it, where you store it, if you've dropped, banged, and if you've broken things in the past.
If you toss it around (at your bed, on your desk/table, at a couch, on a car seat) you should probably use one.
If you store it in your pocket, bag, purse, etc its more likely to encounter dust, dirt, lint, and other objects (keys, coins, etc) then using one could reduce scratches.
If you use stylus (Notes, Zen tablets,etc) it could be good or bad. Having one reduce detection and precision, but it could prevent scratches caused by too much downward force during use.