The Default sales tax rate for the Website, POS and Wedding/Event proposal Manager is set in Settings > Tax Rate.
Using the radio button under the column for POS, Website and Proposal Manager, you can choose which tax rate is applied to each product.
Tax by Line
You can also turn on the Tax by Line feature which will allow you to choose which items on the proposal should receive tax.
How to Use:
1. Turn on on by going to Settings > Event Settings > Tax by Line > set to Yes
2. On the proposal, use the checkbox next to the product to select which items do not get taxed. (If checked, tax will be applied.)