Eh, if I'm doing something like chiseling the shoulders of tenons by hand, I like using a knife to mark that so that I don't stair-step it around the board, plus it makes sure the line that will be visible in the finished product will be straight and not jagged. When marking out for using power tools, I use a pencil, typically a Pilot Sharpwriter. They're cheap as borscht and the spring action they have reduces the amount of lead I break on wood.