I appreciate you for bringing this one to our attention, Jiahui.

We recognize each company has unique needs, and I can see how beneficial it is to your business to place the Tax column at the header level of the customer invoices. However, this option is unavailable in QuickBooks Online (QBO).

When creating a transaction, we need to click the box under the Tax column to collect sales tax for the product or service line(s). For more details about this one, check out this article: Applying Sales Tax to Transactions and Sales Forms. 

For rounding errors, you can update or change the amount manually in the transaction. 

Just in case there's a new tax rate, just update your items and existing transactions to apply the current one. See the Manage a tax rate change article for more details.

I have a link here where you can find articles about managing your sales tax: https://quickbooks.intuit.com/learn-support/gst/misc/02/gst-tax-sg?label=QuickBooks%20Online.

Keep me posted if you need a hand with sales tax adjustment or any QBO related. I'm always here to help. Have a great day ahead.