I don't prewash as well. However, I do wash with shout. (Fabric Dye Catcher). My local LQS owner states that doesn't prewash as well as a retired QS owner states the same. If it's good enough for them - it's good enough for me.
You will find, however, if you participate in any swap, round robin, etc., it is a rule that your fabric be prewashed.