If you're going to be working with the slipped st right away, as in a decrease, if you slip knitwise it lies flat on the dec st. If you're not going to work it until the next row, if you slip knit wise it will be seated as if you had wrapped the wrong way when you get to it, so you need to slip it purl wise. As for the beginning of the row..... I've tried to slip sts, but they come out larger and messier than if I just knit or purl them. However, on some patterns like Clapotis, it doesn't really matter because the slipped stitch is increased or decreased on the following row. I have had better luck slipping end sts when it's the last on on a row - slip as if to purl, then turn and knit or purl the first st. They're not quite as messy for me as slipping the first stitch.