A good set of quality iron sights is always important on any AR-15 that will be for tactical use. It is not a matter of if your optic will fail, but when it will fail. Choose a set of iron sights that allow you to have them mounted jointly with your optic. You may not always need to co-witness (i.e. - if you are running an ACOG or other scope), but you may want to consider sight height when choosing which iron sight works best for you.