Sometimes not, according to information architecture guru, Peter Morville. In a long piece (with several other good nuggets), he argues that a relational model better suits large taxonomies. By organizing vocabularies according into attributes (he calls them "facets,"), you can to string together quite powerful metadata combinations without resorting to complex hierarchical formulations. The downside? Your taxonomy can't form the basis of your user navigation -- which is just the point, says Morville...
Check out Morville's Wine.com example