I would start pinning from the center out and pin on both sides of EVERY matching seam. I know it's a lot of pinning - but hey, we will go the extra step for perfection. I can't imagine that you will have a lot of "play" between the individual squares.
Do you press the seams row by row in alternating directions? I find that can help when you put the rows together as the seams "nestle" into each other.
That said, when I have one of the pieces a little wonky, I mark both pieces in the middle and if necessary in quarters and pin on those marks. That way, the bulk of the "overage" is evenly distributed and matched to an equal amount of stretch on the "short" piece.
Then again.....it may be perfect to start with.