To have a really clean back, I NEVER jump diagonally or horizontally. I sometimes jump vertically, but never more than 4 or 5 counts. Moreover, I count my stitches so that the back only shows tiny vertical stitches (it requires stitching "in line"), and when I finish a color, I tuck the thread under the "Xs" (so on the front of the piece). It's a pain, but this way it's really clean, and it makes my grandmother happy!
(For her if the back isn't as pretty as the front, it's not stitching!)