Even without the adjustable rod (by the way they don't always work) Martin necks seem to keep their relative straightness. The BIG problem with MOST Martins is their neck angle. Folks shave the bridges down over the years but in order to get decent action, almost all of the old ones need neck re-sets. So why don't they change their neck angles? It's all part of the Martin "sound". Seems to work for 'em.
One of the very few current makers who doesn't and WON'T use an adjustable rod is Grit Laskin. Jokingly I once suggested that since many of his guitars now leave the shop with price tags in the twenty Five grand range, he could afford the adjustable rods. At a party once he picked up someone's twenty year old Laskin, sighted down the neck and said "Yup, you can still use it for a straightedge"!