Are Atk Dmg Runes worth it or should I just go Crit Chance/Crit Dmg?
If so about how much of them?

nononononononononononononononoinonononono, never ever never ever ever, get AD flat runes, USUALLY, you want all armor pen for red, armor/magic resist/magic regen for yellow, and CDR/AP/Magic resist for blue, and it varies on Quints

Its Tradition's post outlines decent basic rune setups that favor multiple champs. However for Twitch specifically I think pure crit chance is better than crit damage, but AD is rarely a good setup.

A while ago there was a detailed thread with a logical debate over Twitch's rune setup. I will try to find it.

EDIT: Here is a thread about Crit Chance vs. Armor Pen runes on Twitch.

Twitch Runes