Ganseys don’t have seams. But fake, or mock seam stitches are incorporated into the pattern in most ganseys, and are used to differentiate the front and back sides from each other. They will run up either side of the body, round the gussets, and down each sleeve to the cuff in an unbroken, continuous line. Normally they are just a single purl stitch on each side.
Many of the books have strict principles when it comes to seam stitches. Rae Compton, for example, on page 25 of The Complete Book of Traditional Guernsey and Jersey Knitting, sternly declares, “At best, the seam stitches should flow upwards out of the welt, not simply being increased at the top of the welt as a last-minute addition.” (And she’s right. This is why I’ve stressed how important it is to calculate for the seam stitches all the way through.)
I agree that it looks best if the seam stitches emerge naturally from the ribbing. If you are following a knit 2/purl 2 ribbing, simply continue the very last purl stitch on each side up the body as the seam stitch and the Seam Stitch Police will be satisfied.
I mentioned in another section how easy I find it to miss the seam stitches when I’m not paying attention, and use stitch markers to overcome this. My stitch markers are just a couple of short lengths of a differently-coloured wool, each tied into a loop and secured with a knot or two, with the ends trimmed to stop them getting in the way. I slip them onto the needle immediately after the purl “seam” stitch.
I haven’t missed a seam since I started using them.