I'd be for bypass Cold Iron DR at Tier I, bypass Silver DR at Tier II, and bypass Crystal/Byeshk at Tier III. These bypasses and in this order make the most sense considering what we run into at levels 6 (reavers, renders, flensers in Threnal), 12 (devils in Vale), and 18 (Inspired Quater stuff).
I'd be against giving Metalline at Tier III as it would overlap all metal bypass DRs we got previously thus making them irrelevant.