Instead of viewing solely total sales tax on the job, you can enable visibility of a 'Tax' column in the Estimate screen to populate per line item:
To accomplish this, please refer to the steps below:
1) Go to Tools < System Configuration
2) Search for the user and hit 'Edit'
3) Refer to 'Grid Rights' on the right-hand side
4) Enable both 'Visible?' and 'Allow Edit?' for the 'tax' row
5) Save your changes and log out and log back in