When you're making a decrease that requires a slip stitch, such as SSK or k1sl1psso (knit 1, slip 1, pass slipped stitch over knitted stitch), always slip stitches as if to knit.
If you're working a slip-stitch pattern or a stitch pattern with wrap-and-turn, slip stitches as if to purl.
Not a decrease? Always slip stitches as if to purl unless the pattern designer specifically tells you otherwise.
Why? Because when you slip stitches as if to knit, you're twisting the stitch: changing its orientation from right leg forward on the needle to left leg forward. This tightens the stitch. Makes sense in a decrease.
When slipping stitches as if to purl, you're maintaining the right-leg forward orientation of the stitch on the needle. This doesn't tighten the stitch.
Someone asked me recently if this is written anywhere. If so, I've never run across it. It's one of those little mysteries of knitting that you just pick up. Or by reading this blog.