I can imagine the frustration you're in when tax is calculated by tiny margins, @office-glengray-.
I recognize how relevant rounding off the final amounts of a transaction is. Currently, this option is only available for Indian users. Rest assured that our product engineers are constantly working to meet user needs.
I also encourage you to visit this resource that will guide you in personalizing your sales forms: Customize invoices, estimates, and sales receipts. QuickBooks Online gives you the tools to create attractive, professional-looking invoices, estimates, and sales receipts.
As always, add a comment below if you have any other questions about managing your invoices in QuickBooks. I'm always around the corner to help you. Take care!