I've been looking at recipes for orange marmalade and some say "don't use the pith because it's too bitter" but the photos often show the peel and pith (the part just under the peel) in the final product. And some say just to use the pith and pips in a muslin bag (ie thrown out at the end). Any guidance on this?
I'm posting this as an answer even though I really have very little experience, just some vague stuff I've picked up along the way.
The pith is all about bitterness. Bitter is a big part of the flavor profile of marmalade. So, how much pith you want to use is very subjective. How bitter do you like it? That's the difference in recipes.
Personally, I like maybe 3mm of pith still on the peel that stays in the marmalade.
I can see no benefit at all to using the pips.