Are they worth the extra points just for slightly better armor than a standard warrior with shield?
I would use the IB's as a bodyguard unit for the BSB armed with an Oathstone, not much can hurt it then. Save the points for a big unit of Hammerers!
Unless you're still playing 7E, the Rune of Stoicism doesn't do anything anymore, and the FAQ says:I love hammerers, but they die quite fast, specially since now they always have to fight with great weapons. But step up helps that quite a bit. But as for Ironbreakers, they are a rock hard unit to hold the line. for a standard I always like the rune of stocisim that you mentioned (double unit str) and the rune of Battle for the extra Res.