On your closing question, we would lean toward "neither until asked" for a bare "cup", but only when the source gives no hint at all. If there is any locale signal (the recipe site's language, the page's other units, or the user's own setting), defaulting from that and showing it, as you already do with the "US cup" label, seems like the better balance. A one-click switch next to the result might cost less than a prompt on every conversion. Your point that rounded display values must never feed back into the calculation is one we would put near the top for anyone scaling recipes.