First, you don't say what size the precuts are, so I'm assuming charm packs. I'm thinking that would be a bear to sew after washing. You will have to iron each square, etc. Then, they probably aren't going to be square anymore, plus they would have raveled to some degree. It doesn't sound like sewing them would be a good experience.
All and all, I don't think I'd want to deal with what I end up with after washing. I might wash a couple of squares to see, if you are convinced it will improve the outcome. Then, you will have a better idea of how washing will turn out.
Since it's precuts, it's all made by the same manufacturer. So, I would assume the squares will all shrink the same amount.
I agree, that's a nice assortment.
bkay