How often are Silver, Cold Iron, or Adamantine weapons needed? I am considering going Eldritch Archer at lvl 6 with my investigator. I like the precious arrow feat as giving the potential for damaging otherwise resistant creatures with precious metal arrows. 

My questing is, since I've never had a character higher than lvl 7 in my other campaigns, how often on average would I encounter monsters at later lvls (8+) with these weaknesses? I don't know if this is worth it or would be a waste of a feat.  We are currently lvl 4 in Extinction curse.