Unwashed fabric will look different in a finished quilt if it is laundered when it's finished. It will have a more puckery, drawn look, especially if a cotton batting is used.
If you like that look, you can skip the prewashing, but I never skip the test for color fastness to make sure that nothing is going to bleed when the final quilt is laundered.
I take about a 4" square of the fabric I want to test and just put it into a WHITE bowl (I use a Corelle cereal size bowl) with hot tap water. After about 15 minutes, I check to see if there is any color in the water. It will show up well against the white bowl. If I want to use that fabric, I will wash it with detergent and Synthrapol (to release the excess dye particles) and dry it, then do the same test again. Sometimes one wash will do the trick, sometimes it takes another wash with Retayne (to set the remaining dye particles). After the second wash, if it's still bleeding, I will usually throw it away or use it for a project that is not going to be washed.
Here is a picture from my Webshots album showing what to look for:
http://entertainment.webshots.com/photo/2859530790102889971dbuimu
Check out the previous photo on the Webshots Album to see if you can tell which of the fabrics were the bleeders!