If a customer is tax exempt, should the taxable box be checked and then use a 0% tax code or uncheck the box?

Hello @sgregory-ttsg-co.

Welcome to the Community. Thanks for reaching out with your question about setting up your tax-exempt customer. 

The best practice is to uncheck the taxable box in the customer profile. You could use the 0% tax code, however, if you happen to miss entering that, you could inadvertently charge your customer tax, and then you'd have to correct it. 

I'll include this article to help you learn more about sales tax in QuickBooks Online: Use automated sales tax on an invoice or sales receipt.

Feel free to reach out if you have any additional questions about sales tax or anything else. I'm just a click away and would love to help any time.