Yes, you can set a default discount under your Client or Project page.
First, go to Clients in your Blue menu bar, then from here click on a specific client.
From there, click on the clients Invoice Presets tab and click Edit Presets.
You will see the Invoice Discount which you can either set to Percentage or Flat Fee. Enter in the amount and also set a default to apply to Labor, Expenses, or both. Then click Save when finished.
You will also be able to set this per matter. Click on Matters in your blue menu bar and click on a specific Matter. Click on the Invoice Presets tab and click Edit Presets. Uncheck "Use Client Settings" at the top in order to override the discount for this matter. Click Save when finished.