I would wait until the valve job is done because the back cut on the stock valve, or the nailhead valve, will change the flow numbers and the character of that flow. I always have the valve job done first because it corrects the angles accross which the air has to turn, locates the valve properly in terms of proximity to the chamber wall and in relation to the valve centerline, and takes care of any leakage and inflating of numbers that can happen with worn guides as the valve is run through the lift motion.