I definitely will be switching out the stock nut to a nice polished bone one. I've heard fossilized walrus is very good. Also heard that Bob Colosi's (guitarsaddles.com) West African hard ivory is an excellent choice for a nut. It's Bob's recommendation for a nut. At $45.00 it's a little rich for my blood. I'm fine with a bone nut for half that price. I think bone works just fine; use it for all my guitars. Bob has a vintage bone nut that looks pretty cool for around $25.00 or so. I'm using a bone saddle in my D-25CH.
Anyway, just put some new DR Rares PBs (.012s) on my 25 and polished her up nicely and, man, she sounds and plays great! Not sure my guitar could sound any better. I actually don't think a new nut - bone, ivory or walrus - add much to the tone at all. The saddle is a different story. But I'll be installing a new nut anyway, along with new tuners. Can't help myself, I like to tinker.