This is a burning question for us in Washington state who is strict about use tax. We configured custom fields on our payment request header and line item forms to work with our ERP who calculates sales tax and accrues use tax automatically. But, this is not calculated in Concur. Our suggestion is to build a sales tax table, similar to personal car mileage in Expense, where the client can update the sales tax rates for their local and the correct sales/use tax amount could be calculated in the sales tax field on the payment request header and line item.
In short, we are relying on our ERP for sales and use tax automation with assistance from custom fields on our payment request header and line items.
We are a higher ed client and our ERP is Banner/Ellucian.