Generally, I'm not a prewasher. HOWEVER, I test the fabrics if I even think there is a possibility of color running.
How can I tell?
I've have had years of fabric fiddling. The cottons of my youth all ran. There is a feeling the fabric has that just screams, "this color is going to run." If I suspect it, I'll take a tiny piece, soak in very hot water for a few minutes and then place the scrap on a white cloth. I wring it out in the cloth and check carefully for any color. If there is color, either in the water or on the cloth, the whole things goes either:
Into a hot water soak--with some agitation or to the washer....if it only gets the hot water and looks good, I wring it out in a towel and iron it dry. That has been successful thus far.
I remember a piece of fabric that a friend wanted me to use in a quilt. I washed it five times until it no longer ran a bright obnoxious orange. It was faded but the color was stable!
I always wash quilts when I get them done. Then, I can see if there are any problems. Thus far, it has been OK. But, I tell people about the color grabbers at WalMart.
Last year I got a darling cotton jacket that had red, khaki and white. I was horrified to see red all over the lighter colors when I washed it. I thought it was trashed. So, I got some Rit Color Remover and washed it. I was thinking to dye it black or navy since I really like it. Astonishingly, the only thing that came out was the errant red. My jacket preserved.
I hope this helps......I have some fabrics still that I wouldn't touch without washing. Others, I don't even consider the need to wash.
Oh, any fabric from India, Pakistan or Bangladesh MUST be prewashed and treated......don't even bother with anything else. Their fabrics are not "set" well.
Bali batiks on the other hand, are already boiled and dried in the hot sun. I've never had a Bali run,
Hope this helps.
Happy quilting,
Lenore