Or if you dont have a SS prospect, your next best fielding prospect (CF, 2B, 3B)...
And as far as developing BEYOND projections, everyone's projections are different or vary slightly. So yes they could theoretically improve beyond projections depending on what projections you are starting with HS/Coll vs. ADV and what team the projections are coming from. So it makes sense that you would want to play your best prospect at the SS position to maximize his fielding improvement, if position actual has a quantifiable effect on player improvement