I have done them by hand on a paper wheel knife sharpener on a bench grinder (Dunn System). One wheel has grit embedded on it, while the other wheel is loaded with jeweler's rouge.
Takes a bit of a steady hand and you need to look at what's going on on every pass or so.
My aim is to remove metal evenly along the curved surface until the mark caused by the valve stem end just disappears.
Doing it this way, you can watch the mark disappear evenly along its entire width and length as you go, resulting in maintaining the original shape of the part.
