Hi Mikey
Muddler threads always get my attention, only because I enjoy fishing, not so much so because I enjoy tying them.

Anyway to address your question, I use slips from both sides of the feather. My feathers have sections missing and some left on both sides of the stem. I didn't know going right down one side was a thing to do or not to .

Just courious, why wouldn't you have used from the other side before now?

I use either peacock or turkey quill (depending on my mood) for wings, my fethers usually have long enough fibers on both sides. -Yaf