Have you tried using the "search" feature on the board? Because there have been lots of looooonngggg threads on this topic.
I do not prewash fabric, but I do test suspicious fabrics for colorfastness before using. Shrinkage is generally not a problem if you do a moderate amount of quilting, as then shrinkage is controlled by the batting. In other words, the fabric will not shrink more than the batting shrinks. However, fabric shrinkage *can* be a problem if you quilt far apart or tie the quilt; in that case I would prewash for shrinkage.
Aside from testing suspicious fabrics for colorfastness (drop small piece of fabric in glass of water and let sit overnight to see if dye bleeds into water, then rub damp fabric against white fabrics to see if dye transfers that way), I do a "first wash" of a quilt in a top-loading washing machine or laundromat front loader with Synthrapol and a few color catchers. Home front-loaders typically do not use enough water for this. The purpose is to suspend any unset dye particles in water so they are rinsed away instead of settling into other fabrics as bleeds. This is not enough if you have a bad bleeder fabric -- such as a red that gives up a *lot* of dye -- which is why I typically will test reds and some of the dark colors.
Here are the advantages of not prewashing: (1) saves a lot of time, (2) fabric finishes keep the fabric more stable for cutting and handling. Aside from needing to iron a lot more if you prewash, it's also a good idea to starch the washed fabric to add back some stability for cutting and piecing.