As you have probably figured out, when you slip one at the beginning of a row, it can help keep your edges from getting a loose look - it has a tightening effect because not working that stitch until the next row means it stretches to fill 2 rows of stitching instead of 1.

So if you are simply moving that stitch to the other needle you are doing fine. It shouldn't make much different here, but as a general rule, on a purl row I would slip it as if you were going to purl it, and on a knit row I would slip as if to knit.

And that's all I know about that!