Is it wise or standard practice to back date a credit memo to write off unpaid invoices from previous years?

Hello there, rcw1. I understand how important it is to keep track unpaid invoices from previous years in your account. Let me provide information to help you manage these transactions.

You can zero out the invoices if you have access to make changes to prior, closed periods. Since you're on a cash basis, those entries were never on your balance sheet or P&L so zeroing them out will not impact any cash basis closed period. It will affect any prior period accrual basis reports.   

If you don't have access to make changes, then, IMO, you should create credit memos that offset the revenue on the invoices, not create an offsetting expense. On a cash basis, when you apply a credit memo to an invoice, QB increases your revenue based on the invoice amounts and then offsets that with whatever accounts are assigned to the Product/Services on the credit memo. 

If you assign an expense account to the credit memo, your revenue and expense will both increase, thereby offsetting. That's correct for an accrual basis but not cash. On a cash basis, you don't increase revenue, so the Product/Service on the credit memo should match the income accounts associated with the invoices. That's why it's easier to just zero out the invoices IMO. Forgive me if you know all of this already. 

However, if you want to void the invoices from previous years if these invoices you send through QuickBooks are no longer collectible, you can classify them as bad debts and remove them from your records. This process helps keep your accounts receivable and net income accurate. Check out this article for more detailed information: Write off bad debt in QuickBooks Online.

In addition to that, I'd recommend consulting your accounting professional to help you guide and further check the accuracy of your books.

Furthermore, QuickBooks provides several reports that show the different aspects of your business. You'll only have to customize them to make sure you'll get the information you need most. I've added this article for more details: Customize Reports In QuickBooks Online.

I'll be around in this forum if there's anything else you need further assistance with when managing transactions in your account. Feel free to leave them in the comments below. Keep safe and have a good one!