Add an option under "Tax Settings" when configuring Debit/Credit Types, which will force the project's tax settings to apply to this Debit/Credit (ie, "Use Project-Specific Setting"), and make the "None" (or a "Tax Exempt") setting force the Debit/Credit Type to be always be exempt from the project's tax setting.
My projects all contain both taxable (parts) and non-taxable (labor) items, but most of my projects are also in different tax jurisdictions with differing rates. I'd like to assign the appropriate tax rate to the project, but I do not because currently if I do that, it forces all my non-taxable labor (hourly) items to have the tax applied unless I manually change their tax settings. The current behavior is especially infuriating when adding blueprints, since I have to change the tax setting for each item every time I add one. This has caused me many times to not charge tax when I should have.