Theoretically no. It depends if you want a tree-like structure, or a graph-like structure. A graph is a tree without a head. In general, it's easier and less complex to get started with a tree. But in theory a graph is more powerful. Given that we are just using relations here, there's no strict necessity to have a parent at the head. But the whole concept may struggle to take off without it. So perhaps it might make sense to have a tree structure first, and if it gets used, have a graph as phase two.