Hi Samuel:
If you are looking to have specific default Tax values appear on the "Invoicing" tab for a new customer (like you can with Terms etc), there is no setting for that. It would be a useful addition for an upcoming version because it would reduce errors related to Taxes.

However, if you are using Adagio Invoices rather than OE, there is a Default "Tax Group" setting in the IN Company Profile. It would get used in cases where the Customer's Tax Group is blank.