I plan on starting a WF battlemage, and the Docent from the new Sentinels pack looks like a great fit with DR 5/Magic. However, I was wondering if a lot of mobs' attacks count as magic. Kinds of DR players can get that I've seen include:

DR/Adamantine
DR/Epic
DR/Magic

How common is it for mobs to be able to bypass those?

Also, I've heard alignment-based damage given as a major reason why many builds use the neutral alignment. Just how common and how severe is alignment-based damage? I want to be able to use pure good weapons (don't plan on taking UMD), but I may change plans if dmg vs good is a really big deal.

Thanks