At my store, if it calls for us to pull a casepack of, say, 6, but we only need 1-5 of it, we leave the rest (usually in the box) at the end of the backroom aisle and the backroom people backstock it into wacos.
Lately, though, ever since ePick was introduced I believe, it'll sometimes call for us to pull only 1 of something even though it was in a casepack of six, so it means leaving incomplete boxes on the shelf. The log etl asks us to not leave the 5 or however many items in the incomplete boxes, and I try to abide by that because the backroom people may think it's full when it's not, makes sense. But if it comes from an open shelf and I'm opening a box of, say, tiny ornaments, it may not be appropriate to leave them loosely in their current location because they belong in wacos now, but I don't always have time to open MyWork and remove them from the lower/upper shelf location and backstock them appropriately. And that's what happens with an employee who DOES know how to backstock; I don't think many, if any, of our seasonal SFS workers were trained in MyWork! (I know I've taught many how to check counts in MyWork, so there's no way in hell they would know backstock or the backroom location report.) But leaving it at the end of the aisle obviously isn't appropriate either, since that would mean stuff that's supposed to be in the backroom location isn't there. I agree that the "Set aside any for backstock" isn't ideal, since it removes stuff from the backroom that another SFSer could need (like the damn LOL Surprise cases) and they may not realize it was just at the end of that aisle and INF it.
I'm honestly not sure what would be the best answer to the problem.