The only reason I would do milk first is if I was using the last of it, and would need to add just the correct amount of cereal to it to keep it from being too dry. Normally if I am eating cereal with milk, the cereal and any fruit or other items go in first, and then the milk. However, I eat my cereal without milk most of the time as a snack instead of a meal, so the question doesn’t come up very often.