When I started plotting Etrusca, I thought that I would hide the waist shaping decreases in the sections between two of the slip stitch columns, the ones closest to the sides, so that the adjacent slip stitch lines would curve in and then out at the waist. But I didn't like the way this looked, with some lines curving in and others staying straight, so what I ended up doing was staggering the decreases, one in each garter stitch section (between the slip stitch lines), starting at the sides and moving toward the center, and then staggering the increases in the opposite direction, from the center out.
The resulting curves in the slip stitch lines are very subtle (but nice, I think), but I find it slightly problematic that the decreases and increases are not invisible-- not completely. They show up in the photo as slight aberrations in the garter ridges. To a bothersome degree? Is it better to have the decreases (as in stockinette stitch) be plainly visible, rather than trying and failing to hide them completely? I can't answer that question for all knitters...which is why I'm also including directions in the pattern for doing all decreases and increases in the side garter sections only.