I've never heard of anyone getting just a bolt DLC'ed, but I have a friend who has just his bolt nitrided on his bare Defiance action and it feels really slick.
I had thought of getting the whole action done, however I'd just be removing the DLC when I Cerakoted the outside, waste of money. I think just the bolt would be enough, well at least the portion that runs inside the action, the handle and knob will be cerakoted to match the receiver.