Since you've washed two of the 10" squares, I think you should wash the rest before sewing the quilt top. Otherwise the uneven dimensions will drive you crazy when you try to sew the pieces together.
Depending on how (un)evenly the other squares shrink, you might want to square them up to common dimensions before you begin sewing. I know this will be a pain-in-the-neck, but it will prevent much more serious and harder-to-correct problems later on.
Next time, you might do as Prism99 recommends: Don't prewash. Just post-wash once all the quilting is done (and the binding is attached).