I wash everything except precuts. I want the chemicals out. I want the fabric to shrink before I sew it together. I want to know if it's going to bleed or not.
I figure that precuts are small enough that they are not going to distort the size of a full quilt even if they shrink. I don't use a lot of them but find myself drawn to them more recently and may change my policies if I find my assumptions are wrong. So far, mixing them in with pre-washed hasn't been an issue for me.