DEAR will not create new Tax Rules within your accounting application if you will be using Xero or QuickBooks Online for your accounting, all tax rules should be created within your accounting application which will be downloaded into DEAR. If you do create a Tax Rule within DEAR, this rule will not be synced over into your accounting application.
The correct tax amount will still be seen within your accounting application with an adjustment shown for the difference when you use this Tax Rule on an invoice in DEAR.
Below are images of a rule created in DEAR and used on a sales invoice which was then synced into Xero:
1. A tax rule is created in DEAR from Settings -> Reference Books -> Financial -> Taxation Rules.
A sale invoice is created using the new tax rule.
3. After syncing to Xero, the new rule created in DEAR will not be seen in Xero. Xero will just use an existing rule and create an adjustment to this rule showing the difference as seen in the screenshot below. All invoice totals will remain identical in both apps leaving no discrepancy.