I have made a faux chenille throw and I did not prewash the fabric. Part of the process is to cut between the layers and then wash and dry the heck out of it. Not sure how well it would ravel if the fabric is prewashed.
I made mine with 5 layers. Backing fabric side down and the other 4 piled on top fabric side up. I can't remember how far apart I sewed the strips but I only cut through the top 3 layers. Turned out wonderful.
Important: Make sure to sew on the bias at a 45degree. Someone made one on the QB a little while ago and didn't sew on the bias. The fabric did NOT ravel once cut.