I think work ethic plays a big role for a couple of reasons:
1. If you're going to wait until the Junior season to give minutes to the non-core categories, you probably aren't getting that 20 points of development unless the work ethic is above average.
2. If the work ethic is high enough, you can put zero minutes into a category and you typically won't lose much. Sometimes nothing at all and typically it is just a point or two. Whereas a low work ethic player can easily lose 5+ points without minutes.
A couple of other things I would be considering:
1. What's the starting category? As a general rule, I will protect a category like Weena mentioned if the absence of minutes may cause it to dip below 10. It's tough to get single digit ratings to increase and if you plan on eventually developing the category, you want to avoid the single digits.
2. Where is he on the depth chart? If he's getting minutes as a post player, then I'd be much more inclined to focus on cores. A guy like you describe might be #3 on my depth chart for SF however. So I'd probably focus on the non-cores since he'll be getting some minutes at his non-core position. And if he's buried behind some upperclassmen, I'd also give minutes to the non-cores.