Folded EOs, like 5x, 10x or 15x Orange, etc., probably don't need anything to anchor the scent down. I have some 15x Tangerine, I think, and it smells pretty good.
The EOs like Pink Grapefruit (*drool* smells SO good) which aren't super concentrated like the folded EOs, some people use cornstarch, others anchor it with another scent like patchouli, neroli, something deep and earthy. Unfortunately, I don't know the amount of cornstarch to use and haven't searched for it myself.
I am too cheap to use my very few EOs in a soap batch right now. Maybe after I make the castile I have planned, I will work on a recipe to make via HP and use my EO in that. I'd LOVE to have pink grapefruit soap, it smells that delicious!