I'm not a expert on how spree handles taxons, but my basic understanding is this:

* For each `Spree::Taxonomy` there is a `Spree::Taxon` whose `parent_id = NULL` and has the same name as the corresponding `Spree::Taxonomy`
* MailChimp only allows you specify one category. Products in spree can be associated with a taxonomy with many parents. Each case is different, but I found it the most helpful to put products in the category in MC right below the root taxon. So if a product was in "Categories --> Clothing --> Men --> Shirts" the category in MC would be "Clothing"