I prewash.............that being said, it comes from experience of having colors bleed, then your quilt is ruined.
I've always "prewashed", course that didn't help a quilt that was a wedding present that I did. It took some soaking and OXY clean to get most of the blue of the yellow and the yellow out of the white. And these two fabics were washed/dryed twice before the cuttin' and quiltin' began.
I pre-wash panels and strips(you wouldn't believe the colors that bleed) in a big bowl of HOT water and then rinse in cold, roll up in a towel to get most water out and throw in dryer with towel and extra dry towel to assist in drying quickyly. Have very little strings from edges being a problem this way too.
Another thing to add is the "Color catchers". These work wonderful, but I still prewash. After the blue/yellow fiasco, I learned my lesson. Not taking chances.