I prewash all yardage and remnants. I haven't used precuts yet, although I do have some. I will wash them before using also, when I have time. I find there is often enough shrinkage that the cut ends can be off by as much as 6 inches once dried. That alone tells me to wash everything.
I can't imagine giving a quilt as a gift and having the recipients totally disappointed once they wash and dry it, only to find that the quilt either shrunk or that the dyes in the fabrics have run. How embarrassing! I want to feel confident that they will have no issues with it after all my hard work.