I'm not sure how much difference that would make in the grand scheme of things. Wizards would make a rather small percentage of the population, and learning to read could be one of the first steps for a young apprentice wizard-to-be. Medieval Europe had scribes and scholars and such despite being rather low in literacy overall. The real issue is going to be what is happening out in the farms and villages and would that increase or decrease literacy, since the bulk of the population is out there in the farms and villages.