I wouldn't prewash anything already pieced. I speak from sad experience, the seams ravel like mad.
I've been quilting since 1971, and because i always wash new clothes before wearing them, I just also prewashed my fabric. I run a sink of hot water, smoosh them around, wring, shake and pitch in the dryer; fraying is next to nill
. No unwashed fabric enters my quilt room, and then I don't have to wonder, did I or didn't I.
I have trouble breathing freely in a fabric store, I don't want all that treatment stuff in my quilt room.
Many folks here are absolutely on the non-washing side of the fence, opinions are free
. No rules...