Hey @pklitscher, weclome! 
The is_taxable field on a CatalogItem will determine whether taxes are automatically applied when creating an order with "auto_apply_taxes": true. Specifying taxes on the CatalogItem itself will determine what taxes apply to it by default, but that can still be overridden at the time of sale.
Hope that clears things up! Let me know if you have any follow up questions.