Take time to think through your ideal organizational format before creating your tree.  For example, a collection of Western Native American Headdresses could have just one node with several different nations represented in the one Collection page.  Or it could have separate subnodes for each nation, each with its own collection.  Either way works and is likely dependent upon the size of the collection.  The larger it is, the more likely you'll want more nodes on the tree to represent different elements of the collection.