You have the ability to assign one or more of your tax rates/fees as a default tax rate. This allows you to auto-assign this tax/fee to all new add-ons and products that you create saving you time with your inventory set up. To do this simply navigate to Settings > Tax Rates and Fees and either edit an existing or create a new tax rate/fee.
Check the box called Default Tax Rate/Fee and click Save Changes. Going forward this tax rate/fee will automatically be assigned to all new add-ons and products created. You still have the flexibility to remove this tax rate from the new products if you so choose.