Well, I can't say it applies definitely to all Scions, since the term isn't really used in the books and so I don't know for sure whether you only count creatures truly born of two (sharing genetics) races or if you include things like White Court virgins and similar.
If you do, then we know at least the Red Court Infected and White Court Virgins make a choice. Similar is the case with Denarian "infected" (those with a coin and Shadow but who haven't let the demons into their heart).
If you're talking about other things I actually can't think of any half-critters that are not of fae origin that also don't fall into the category idescribed.
That said, a lot of the recent writing in Dresden seems to be around the power of free will and choice, so I expect Jim would write that anything half human comes with the ability to choose baked in.