I used to run full ArmPen runes on my AD carries since they scale into later game better. With the current afk farming meta down bottom, I understand why it is nice to get flat AD instead, so that you can farm minions better.
The thing is, I already can farm minions just about as well as if I had the bonus 15 AD. What makes me unsure of which runes to run, is that if I don't run flat AD and the other carry does, I lose all trades with them and can get bullied out of lane. If you are against a Graves or Corki and they have the +15-19 AD from runes, you can't even attempt to trade with them if you are Ashe. When I do try, I end up failing and having to hide under tower with whoever my healer is.
If you can give me tips on what to do, that would be helpful. Really the only option I see is to run flat AD, so I can at least attempt to trade with them instead of getting stomped on.