I once saw a grinder attachment for resurfacing the rocker arm to valve face. It had an articulated arm that preserved the face shape. I do not have one of those. I do use a stone(knife sharpener type) to clean up those faces. And I take care to keep the profile as close to the original as I can. The issue that might come about is if the outer tip is damaged. It would be unwise to shorten the rocker. BDammers47 has posted why. If the top of the valve stem is worn(seen that too) the valves are not so expensive as to make it worthwhile to trim that. Just replace.
