I usually prewash, if I get it done in time. Some of my baby quilts I'm finishing the stitching on the label just a few minutes before heading out the door. It ensures that I've left no germs or loose threads on the quilt. Plus it gives me the assurance that the quilt will withstand being washed, the seams are secure, and that the fabrics shouldn't bleed. I wash just like my normal laundry - cold water, regular cycle, dry on hot (my dryer is :-( and only has "cool knit" or "hot regular"). I use my normal detergent (either all or purex "free and clear").
Ooops, just realized this was "pre-washing fabric", not "pre-washing quilts prior to giving" - I don't prewash fabric purchased at stores. If purchasing from someone's stash, I do sometimes prewash due to possible animal hair and smoke.