Pre-washing isn't a must. Lots of people don't pre-wash. It's a personal choice and there are lots of reasons to pre-wash, and lots of reasons not to. There is no right or wrong way (although I think most will agree that pre-washing flannel is a must ... but I digress).
I happen to be a dedicated pre-washer. BUT I wash yardage as soon as I bring it home. If I bring home fat quarters I wash those in the kitchen sink by hand then machine dry.
I would not pre-wash anything smaller than a fat quarter, and I would definitely NOT pre-wash anything I've already cut for a quilt. The risk of it shrinking unevenly (fabric shrinks more along the WOF than it does along the LOF) is too great, as well as the risk of it being skewed on the bias.
I would also not prewash the finished quilt tops ... for the same reasons. If it's square now, it may not be when you've washed it.
If you are worried about fabrics bleeding you can use one of two products in the wash: Retayne will retain the dyes and "set" them so they don't bleed. Synthrapol is another product that I use along with Color Catchers and these two products are designed to allow the fabric to bleed but the Synthrapol will hold the dye in suspension and the Color Catchers will pick up the dye. I will then continue to wash with clean Color Catchers until the Color Catchers come out without any dye on them.