marking eggs with graphite pencil

is this for when they go into an manual turn incubator or just for in your kitchen?

then no need to mark them if they are to eat - I use an egg skelter so the oldest one is always at the bottom 
this is what I am on about 

Nearly every breeder I know uses graphite pencils on their eggs; and these breeders win classes at the National and the Federation.

I know some people use china graph markers, and I also know some people use felt tips (though I wouldn't personally)

I also use graphite on my eating eggs.

I don't think I've poisoned a chick yet