The answer is both. There is a lot of science involved in a wheel design. The spoke pattern, hub design etc. are all developed using engineering principles to build a strong lightweight (and good looking) product. However to put all of the components together to make a strong wheel is more of an art. If each of the components were manufactured to be identical and perfect then a robot could be programmed to repeat the building of perfect wheels. However there are manufacturing inaccuracies that make a robot built wheel a compromise. The wheelbuilders art is to gradually build a wheel that is as true as the components allow and ensure each component, especially the spokes are all doing their fair share of the work. An ability to visualise in 3 dimensions what is happening when you tighten a particular nipple is required. Each action in building has a knock on effect, tightening one nipple will increase or decrease tension in others so the wheelbuilding process is a gradual refinement that works towards the goal. Much patience is required to develop the feeling for what to tighten and by how much, this “feeling” and viualisation that wheelbuilders develop is very much an art.