You can exclude the sales tax to your customers. Please look at top left of any customer profile screen. You’ll see a “Non profit” option. Check the box and make sure to click “Update” at the bottom of the screen. That customer will never have sales tax added to their invoices or estimates.
And by doing this, then you never have to worry about changing sales tax manually (between “yes” and “no”). It will assign no tax automatically. If you have any questions, please let me know. Take care and have a nice day.