Ah yes, to slip or not to slip, the perennial question...
Each has it's advantages, which I'm still learning about. Apparently, not slipping can make for the nicest, most invisible seams, on medium weight yarn. I've yet to try this.
I have always slipped, but I may change my mind when I experiment more with seaming and not slipping. Also, I want to experiment more with picking up stitches on a non-slipped edge. You can pick up stitches quite nicely on a slipped edge, if done properly, but it can be a neusance to find the proper hole to pick up alternate stitches in, unless you're picking up only one stitch for two rows, then it's a breeze.
For stockinette selvedges that won't be sewn, I definitely like the slipped edge look.
It's really a personal preference. People swear by both!
For that pattern, with that fuzzy yarn, slipping makes perfect sense, because it's easier to sew up, and you won't notice a gap from only doing one stitch every 2 rows.