"when would i want to have the overscan turned on?"
When watching TV or a badly authored DVD which has noise or other crap around the edges or the image doesn't quite go to the edge so you get a small black border. I find most TV stuff to be fine these days(using Sky) and have overscan turned off 99% of the time.
__________________ Projector:Sanyo Z4 CRT TV: Panasonic TX-32PB50 A/V Reiciever: Pioneer VSX-D812 DVD: Samsung 850 Speakers: Kef Q Fronts and Gale Centre, Sub and Bi-Polars. Xbox360 and Sky HD. Harmony 688 remote
i've read that overscan is partly responsible for overfuzziness too, so best to leave it alone unless you have "crap around the edges or the image doesn't quite go to the edge so you get a small black border" as paul_mowbray rightly points out
my sp5000 used to look better with overscan on, but not so for the 900 apparently