I just started sewing all the squares together last weekend (no,they're not all done yet--got 27.5 out of 30 done) and have been learning some lessons in the process.
Lesson 1: Always slip the first stitch of each row when knitting an entire square, and never do when you're knitting a section of a square (like the border on Kathleen T. Carty or the center on Betty Salpekar). OK, I don't actually know if this is a hard-and-fast rule, but the squares where I conscientiously slipped the first stitch seam up beautifully, with the ridges appearing unbroken across the two squares. And when I attempted to do the same thing on the sections they came out like crap. Might just be me though. =)
Lesson 2: When the ridges on both squares run perpendicular to the seam, use a "ladder stitch" on the FRONT side of the work (see "Assembly" diagram on the last page of Knitter's School in the back of the book for a picture of what I'm calling "ladder stitch"). When the ridges on both squares run parallel to the seam, use an overcast stitch through the FULL THICKNESS of the squares. My first bunch of parallel seams look terrible because I was trying to only seam on the front side. And, finally, when one block runs parallel and the other is perpendicular--err, make it up. I tended to use overcast on those too, and it looks fine, but some experiments with half-and-half suggest that might be the way to go.
Hopefully this'll save some of you a little bit of stress with the seaming.