117becca - I think we share a brain! Same here - wash when completed. But I also prewash all my fabrics as I mostly use yardage. I don't wash precuts though. But by the time I've hand quilted something, the dog hair, dust, etc. make it necessary to wash it before gifting. And I get the crinkly look I love.
I would say if you don't prewash your quilting fabrics, there's no need to prewash the muslin.