I read the washing instructions that came with my charms and they say not to prewash...I guess it might mean they have already done that?

So far, I have never had any problems with anything made from pre-cuts...

I usually prewash dark colors or hand dyed fabrics, but rarely do I prewash newer cottons...they seem to be processed differently than the older stuff...I would err on the cautious side if you don't like to be surprised in a bad way...