If you would like to submit a proposal with no tax, you can do so within the proposal setup as shown below.
Scroll down to the Profit & Pricing Breakdown section at the bottom of the proposal, then select the pencil icon top open edit mode.
Enter the tax rate as 0%. Be sure to leave parts or labor (or both) checked when you change the tax rate to 0%. Then, select the orange check mark to the right of the 0% to save the changes.