I've got a dozen or so checkering files (for spacing) but am as likely to use a thread-chasing file as one of the "real" ones. Thread-chasing files are (relatively) cheap, so I can grind them down with a relatively) clear conscience. Once layout is done, I use a triangular file to cut the checkering. Basically, it is like doing checkering on wood: layout, spacing, then cutting the lines/pointing up.
Except it will look a lot better with a border (actually, thinking about it I prefer a border on wood checkering too). The problem is doing the lines up to the border--a file doesn't work for this. I use an engraving tool, a friend uses a die sinker's chisel (and he's much better at it than I am), another claims bordered checking on metal is the root of all evil so he doesn't do it.
LPI depends on what you are doing. You know how wood checking loses the grippiness if it gets too fine? Checkered metal gives a good grip with much finer checkering. For example, a revolver hammer might get checkered at 50-60 lpi, and still give a good grip. Something as coarse as 20-35 looks out of place, almost like a caricature, on a hammer.
Having expended all that blather, I'll point out that if you hook your finger under the lower jaw of a flintlock, you'll have much better control that trying to manipulate the hammer spur. YMMV, of course.
