I have seen a few requests and a couple of mods that handle something like this, but here's my simple version:
Our client offers wholesale discounts at 3 levels. Each level has the word "Wholesale" in the name.
In includes/modules/order_total/ot_group_pricing.php around line 70 add this, right below the relevant db queries:
then in includes/functions/functions_taxes.php around line 15, at the beginning of function zen_get_tax_rate, add this:
// added to make wholesale groups tax exempt. See also includes/functions/functions_taxes.php
You could use a different string where I have "Wholesale", like "Tax Exempt". Any group with that string in the name is charged 0 tax. I have not tried this with a group that does not already receive a discount.