Consider:

• the extent to which those records are organized and accessible outside of the accounting system
• how much is at stake if a deduction is challenged
• how easy it is to get the organization into an app like Entryless or Expensify or any one of a ton of others in which the spender snaps a photo of a receipt and sends it in for OCR and classification
• the volume of transactions that could be material that could potentially be challenged, individually or taken as a whole
• whether anyone is doing quality control on the data entry and needs quick access to the backup for classification
• the likelihood of the organization to convert to a different accounting system
• whether the organization has an annual independent audit

Our clients in the $1M-$10M range get us backup documentation for pretty much every transaction. And it comes in handy a LOT.