is almost always best done after the finish is completed. For one thing, it makes it MUCH easier to mark out your layout lines and cut your first rows. It's hard to see well on unfinished wood. Then after your diamonds are pointed, work a little finish into the checkering with a toothbrush. Voila!
I've checkered a number of rifles and pistol grips on newly purchased guns. Lots of fun. Gets tough on the eyes when working on multicolored laminated stocks though...hard to see your lines.