Just to be clear, you'll reverse the pattern of knits and purls on the alternate rows if you're knitting the piece flat (not in the round). I assume you are, but that's an important distinction to make.
As I understand it, when you slip a stitch it's always slipped as if to purl unless it's part of a decrease or the pattern specifically says to slip it as if to knit. There may (probably ARE) exceptions to the exceptions to the rule too. I'm not aware of any purlwise decreases that involve slipped stitches, but there are a LOT of things I'm unaware of. There are far more experienced folks here to answer that question. But chances are that if you encounter something like that in a pattern, it will be sufficiently unusual that the pattern writer will give instructions on it.