More pattern information for your exact pattern might be helpful. But the basic idea is to decrease what you need to and then if you have "eaten into" one of the cables so that you don't have enough to work the cable anymore, then work the stitches in the background stitch until you get over far enough to work a full cable again.
As far as working the decreases in a bit, I usually like to. At least one stitch in is nice, but if it seems easier for a certain project I might do them right at the edge. If you want to work them in a stitch or two just do like I said above but keep the 1 or 2 stitches on the edge the same all the time and not involve them in any changes.