What nobling barbs is better then minting?
On no hauls - Nobling a village, any village, is better than keeping lots of nobles held back in reserve doing nothing.
We lose nothing by nobling them, as they can't be farmed. And even the small resource input is more beneficial than keeping a noble sitting at home.
People should always maintain enough nobles to participate in any tribal operations that crop up, but any spare nobles beyond that, you may as well noble something with them.
Once all the small players have been nobled up, and you've only got real tribal targets remaining, your rate of expansion is not generally going to be constrained by the rate that you mint coins - it'll be constrained by your nukes, diplomacy, or the rate at which you can organise large scale operations.
tl;dr version: Nobling barbs with spare nobles is fine.