You really couldn't do coining very well with a mill, outside of CNC. These fine little embellishments are best done by hand. Checkering files work okay on some shapes and some materials and not so great on others. I have some checkering files and use them sometimes depending on what I'm doing. I bought a 20 LPI and a 30 LPI. I use the 20 most but the 30 has it's place on real fine stuff. I can't imagine the need or desire for anything smaller than 30 LPI for what we do with knives.
Checkering files aren't a magic cure all and they take as much skill with a file as anything else. Trying to roll a checkering file around a round or oval spacer and keeping the file flat to the edge of a tiny spacer and perpendicular with it's edges so the lines don't go crooked, all while only having a tooth or two engaged takes practice and skill. Some materials, like bronze or nickel silver or copper that are really soft, are REALLY easy to roll the teeth of the coining over, jump teeth and ruin the whole piece. I've done that more times than I can remember.
Also, with a checkering file you have to be really lucky to have your starting point and finishing point end up the same space apart so you don't have this odd big gap, or two teeth crammed really close together. That looks bad. It usually doesn't work out very well for me with a checkering file.
The way Longdog mentioned that Stacy Apelt does it is a good way, and very similar to what I do.
I prefer needle files, good quality ones at that. I paid over $120 for a set of 5 and I'd do it all over again. With this kind of work, there is no substitute for good needle files that are straight, true, hardened properly and have nice fine sharp teeth all the way out to the tip of the file.
When my spacers are all shaped, I take a thin strip of paper and wrap it around my spacer and mark the end with a fine pencil point so I know my circumference of the spacer. Then I straighten the paper out, measure it and divide it by equal increments, whatever spaces I want the teeth on my coining to be. usually 1/16ths work out pretty well. If not, you can move to the next increment or slightly shift several spaces to make it work so it's not noticeable. I mark out ever 1/16th on the paper then I have a little flexible ruler of sorts. I color the edge of my spacer with sharpie marker and lightly mark each spot on the ruler with a sharp tip scribe. Then I use a small triangle needle file and begin cutting each tooth with the edge of the file.
It takes a little time and some care but with a good file and some practice, it becomes fairly quick and fairly easy. Maybe I can get some pictures the next time I'm doing some.
That's just how I do it most of the time. I'm sure someone has a better faster way but this works for me.
Things like this though are why I don't post a finished knife every couple of days. I trust my hands and my eyes way more than I do my machinery and I find that as I improve as a knifemaker, I come off the machines earlier and earlier and do more finish work by hand. I like the results that I get that way much better. But as a result, I get slower the more I improve.