before the fact - you can prevent curling at the side seams by knitting across the right side rows as given in your pattern, and on the wrong/purl side row, slip the first and last stitch purlwise (or knit the first and last stitch, and purl all the rest, creating a one-stitch-wide garter stitch column up the sides.) Your work will lay so much flatter for you.
after the fact, yes, you will have to work it all out in blocking. you'll have to shape and pin to the correct measurements. and then seam after having blocked the pieces.
I hope this helps out - if not this time, then next.