Hi, First of all, you might find a problem with the rack length capability of the prospective laser cutting company.
The other problem might be distortion when the material is cut from one side of a piece of steel, causing the rack to bow so needing straightening, which means any accuracy you might think is there is probably as good as measuring your work with a "very good tape measure'.....LOL
Anytime you apply heat to metal you initially get expansion at the site of the cut which increases as you prolong the agony, and then differential contraction as the part progressively cools.
So even if'n you get the rack cut from a piece of plate stock, in the middle, cut all round, as opposed to cutting the teeth only on one side of a strip of metal, the heat movement will destroy any accuracy you require.
Why'nt you just get module gears to suit the off the shelf machine cut racks, much cheaper and far more accurate.
I suppose if you want to be different you could always get the racks wire cut, might be pricey.