It seems more logical to put the cereal in first otherwise you probably will waste a lot of mile if you overpour. I, however, like my cereal soggy so I put the cereal in first and give the milk a chance to soak in before I eat any. If I liked it crunchy I would just eat a handful of cereal dry and skip the milk part completely because that is the only reason for the milk really..
This is pretty much the same argument about toilet paper, Over or under, when the obvious answer is over. I have had guests visit and when they used the bathroom they switched it around to under. I think that's fairly over the boundary of the rights of guests.