I started out as a pre-washer for years. Now I don't prewash. It's a great time-saver and, at my age, I prefer to use my time making quilts than washing fabric!
I do test any fabrics that I suspect might bleed, such as the red fabric in the photo above. I drop a small piece of fabric into a glass of water and leave it for several hours to see if any dye bleeds into the water. Afterwards I rub the damp fabric against other light-colored fabrics to see if dye transfers that way. (Some fabrics pick up loose dye easier than others, so it's a good idea to test several.) If there is a lot of dye coming out of the fabric, I will prewash it (with Synthrapol unless it's a solid fabric) to see if it stops bleeding.
I do not worry about fabrics that bleed just a little bit, because Synthrapol and water in the first wash will take care of those. It is the fabrics that bleed a *lot*, like the red fabric in the photo, that I want to take care of before cutting. I don't bother with Retayne anymore. If the fabric continues to bleed after two washings in Synthrapol, I don't use it in a quilt as most likely the dyes were not set properly in the factory. Fabrics that stop bleeding were simply over-saturated in dye and not rinsed sufficiently at the factory; I can take care of that at home.