Generally speaking, from which transaction should a Return Authorization be created against? Sales Order or Invoice/Cash Sale?

If fulfilled, not invoiced, then SO. If fulfilled and invoiced, the Invoice. If sold in cash, then cash sale

Thanks @Merk Are there any best practices reasons as to why the invoice would be preferred over the SO if billed?

We do SO only.

Can't tell why from invoice over SO. The only things differentiate the invoice from SO is a) there are multiple invoices for a SO and b) invoice is the last transaction in the sales chain. It also to do with the role's permissions. A customer wants to return. Some companies give users to view/edit invoice, so they can create RMA from invoice. Some only grant permission to SO. Therefore the user has no choice but to create RMA from SO.