While the accuracy is needed at the line-level, the request here is to allow the branch or company or external tax provider the control to either (a) submit requests "manually" or (b) wait to submit the tax call one time when saving the document (this would apply to sales orders, quotes, and invoices).
The purpose behind this request is that Avalara charges are designed to compare based on a 10:1 ratio "GetTax() calls" versus "Committed Documents (invoices)". Whichever is higher serves as the basis for the charge. In other words, each 10 calls makes for 1 committed document. The way Acumatica sends these GetTax() calls today makes for a very expensive relationship with Avalara.
This particularly affects companies with many document lines.
While other systems such as Oracle and SAP do the same, other systems like Sage 100 and others make for a less expensive relationship with Avalara.