From what I have read, the old-fashioned dye-setting chemicals such as vinegar and salt that worked on natural dyes don't work anymore because fabrics are almost all dyed with chemicals now.
Do you have a concern about a specific fabric? People who prewash their fabrics (not all quilters prewash) typically just use a mild soap and wash as usual. Orvus is a soap specifically made for washing quilts, but I don't think most quilters use it also to prewash fabrics.
Retayne sets unset dye in fabric and is suitable for setting home dye jobs, one-color fabrics that you think might run, and fabrics you know are running.
Synthrapol suspends unset dye particles in water so they can be rinsed away instead of settling in other fabrics (bleeds). It is often used the first few times a quilt is washed. (You would never use Retayne to wash a quilt, because it would set any unwanted bleeds.)
Most fabrics these days are colorfast for washing. Most fading comes from exposure to sunlight or other light sources (including flourescent lighting). Fabric is manufactured to meet minimum requirements for exposure to light before fading. Not sure what they are right now, but actually they're quite minimal. In the interests of safety, some chemicals that were better at preventing fading are no longer used.
Sorry if this is more info that you were looking for! :roll: